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VoIP Security: Common Threats and Best Practices

VoIP has transformed business communications by allowing voice calls to travel over IP networks instead of traditional telephone networks.

However, because VoIP relies on internet connectivity, servers, SIP signaling, user accounts, devices, and network infrastructure, it also introduces security risks that businesses need to understand.

A compromised VoIP account can potentially be used to make unauthorized calls, access voicemail, intercept communications, or disrupt business operations.

The good news is that most VoIP security risks can be significantly reduced through proper configuration, strong authentication, network controls, monitoring, fraud protection, and regular security practices.

In this article, we’ll look at the most common VoIP security threats and the best practices businesses can use to protect their communications.

Why Is VoIP Security Important?

A VoIP system is more than just a telephone.

A typical business VoIP environment may include:

  • IP phones
  • Mobile applications
  • Desktop softphones
  • SIP accounts
  • Voicemail
  • Auto attendants
  • Call forwarding
  • SIP trunks
  • Internet connections
  • Firewalls
  • Session Border Controllers (SBCs)
  • Cloud communication platforms
  • User portals and administration interfaces

Each component needs to be properly secured.

A security problem affecting one part of the environment can potentially impact the entire communication system.

For example, if an attacker obtains a user’s SIP credentials, they may be able to register a device or application using those credentials and potentially make unauthorized calls.

This is why VoIP security should be considered as part of the overall IT security strategy.

Common VoIP Security Threats

1. SIP Credential Theft

One of the most common VoIP security risks is the theft or compromise of SIP credentials.

SIP credentials typically include information such as:

  • Username
  • Authentication ID
  • Password
  • Domain or server information

If an attacker obtains valid credentials, they may attempt to register a device or softphone and use the account.

Depending on the configuration, this could allow an attacker to:

  • Make unauthorized calls
  • Receive calls
  • Access voicemail
  • Modify call settings
  • Forward calls
  • Generate significant telephone charges

How to protect against credential theft

Businesses should:

  • Use strong, unique passwords
  • Never reuse VoIP passwords for other services
  • Avoid sharing SIP credentials
  • Change compromised credentials immediately
  • Protect configuration files containing SIP credentials
  • Disable unused accounts
  • Monitor unusual registrations

2. Toll Fraud

Toll fraud occurs when an unauthorized person uses a telephone account or VoIP system to make calls that generate charges.

This is one of the most financially significant VoIP security threats.

Attackers may target accounts that have access to:

  • International calling
  • Premium-rate destinations
  • High-cost mobile destinations
  • International toll numbers
  • Other expensive routes

An attacker may make hundreds or even thousands of calls before the activity is discovered.

How to reduce the risk of toll fraud

Businesses should consider:

  • International calling restrictions
  • Destination restrictions
  • Calling limits
  • Spending limits
  • Time-of-day restrictions
  • Fraud alerts
  • Real-time call monitoring
  • Automatic blocking of suspicious activity

Businesses should only enable expensive destinations when they are actually required.

3. SIP Scanning and Brute-Force Attacks

Attackers and automated systems can scan the internet looking for exposed SIP services.

They may attempt to discover:

  • SIP servers
  • SIP ports
  • Extensions
  • Usernames
  • Authentication endpoints

Once a target is identified, attackers may attempt repeated authentication attempts using guessed credentials.

This is commonly referred to as a brute-force attack.

Best practices

To reduce this risk:

  • Use strong passwords
  • Limit authentication attempts
  • Block suspicious IP addresses
  • Monitor failed registrations
  • Avoid exposing unnecessary services directly to the internet
  • Use firewall rules and SBC protections where appropriate

4. SIP Registration Hijacking

SIP registration tells a VoIP system where a user’s device can be reached.

If an attacker obtains valid SIP credentials, they may attempt to register their own device using the victim’s account.

This can result in calls being directed to the attacker’s device instead of the legitimate user’s phone.

It can also be used to make unauthorized outbound calls.

Warning signs

Possible indicators include:

  • A phone unexpectedly stops receiving calls
  • A user reports that their phone is intermittently offline
  • Unknown devices appear in registration records
  • Calls are being placed without the user’s knowledge
  • Multiple registrations appear from unexpected locations

Monitoring SIP registrations can help identify this type of activity.

5. Call Interception and Eavesdropping

Voice communications can potentially be intercepted if signaling and media are not properly protected.

There are two major components to a VoIP call:

Signaling controls how the call is established.

Media carries the actual audio.

Depending on the VoIP architecture and available features, encryption can be used to protect signaling and/or media.

For example:

  • TLS can be used to protect SIP signaling.
  • SRTP (Secure Real-time Transport Protocol) can be used to protect voice media.

Encryption availability and implementation depend on the VoIP platform, devices, network configuration, and interoperability requirements.

Businesses handling sensitive communications should understand what encryption options are available in their specific environment.

6. Voicemail Attacks

Voicemail is another component that should not be overlooked.

Weak voicemail PINs can allow unauthorized users to access voicemail accounts.

For example, easily guessed PINs such as:

  • 1234
  • 0000
  • 1111
  • A user’s extension number

should be avoided.

Voicemail security best practices

Businesses should:

  • Use strong voicemail PINs
  • Avoid predictable PINs
  • Change default credentials
  • Limit failed authentication attempts
  • Disable unused voicemail accounts
  • Review voicemail access settings
  • Be careful with voicemail-to-email configurations

7. Call Forwarding Abuse

Call forwarding is a useful business feature, but it can also create a security risk if an account is compromised.

An attacker who gains access to a user’s account may attempt to forward calls to an external number.

This could potentially result in:

  • Unauthorized call charges
  • Business calls being redirected
  • Sensitive information being exposed
  • Customers being redirected to an unauthorized destination

For this reason, businesses should monitor changes to call forwarding and other important account settings.

8. Denial-of-Service Attacks

A Denial-of-Service (DoS) attack attempts to overwhelm a system or network with traffic or requests.

In a VoIP environment, an attack may target SIP services or other communication infrastructure.

The objective may be to prevent legitimate users from:

  • Registering their phones
  • Making calls
  • Receiving calls
  • Accessing communication services

Proper network architecture, firewalls, SBCs, traffic controls, monitoring, and provider-level protections can help reduce the impact of these attacks.

9. Malware and Compromised Devices

VoIP security is not limited to the VoIP platform itself.

A user’s computer or mobile device may also be part of the security equation.

For example, if a computer running a softphone becomes compromised, an attacker may potentially gain access to credentials or other sensitive information.

Businesses should therefore maintain normal endpoint security practices, including:

  • Operating system updates
  • Security software
  • Strong passwords
  • Multi-factor authentication where supported
  • Application updates
  • Secure Wi-Fi
  • User security awareness

10. Social Engineering and Phishing

Attackers may attempt to trick employees into revealing credentials or changing account settings.

For example, an attacker could impersonate:

  • IT support
  • A telecom provider
  • A manager
  • A customer
  • A technical administrator

The attacker may ask the employee to provide a password, verification code, or other sensitive information.

Protecting against social engineering

Employees should be trained to:

  • Never share passwords
  • Verify unexpected requests
  • Be cautious with links and attachments
  • Confirm requests for account changes
  • Report suspicious activity

Technical controls are important, but user awareness is an equally important part of security.

The Role of the Firewall

A properly configured firewall is an important component of VoIP security.

Firewalls can help control which traffic is allowed to enter or leave the network.

Depending on the network architecture, firewall policies may restrict:

  • SIP signaling
  • RTP media
  • Management interfaces
  • Remote access
  • Unnecessary inbound traffic

However, VoIP can be sensitive to incorrect firewall configurations.

Overly restrictive rules can cause problems such as:

  • One-way audio
  • No audio
  • Failed registrations
  • Calls that cannot be established

Security should therefore be implemented carefully while maintaining the connectivity required by the VoIP service.

What Is an SBC and Why Is It Important for VoIP Security?

A Session Border Controller (SBC) is a network element designed to control and protect VoIP communications between different networks or communication environments.

An SBC can provide functions such as:

  • SIP traffic control
  • Access control
  • NAT traversal
  • Traffic filtering
  • Topology hiding
  • Rate limiting
  • Protection against certain types of SIP attacks
  • Call routing and policy enforcement

SBCs are commonly used by telecom providers, carriers, UCaaS providers, and larger organizations to help secure and control SIP communications.

For businesses using a managed or hosted VoIP service, many SBC functions may be handled by the service provider.

Protecting VoIP Accounts

User accounts should be protected just like other business applications.

Recommended practices include:

Use strong passwords

Avoid simple passwords and never reuse VoIP credentials for other systems.

Disable unused accounts

Old employee accounts, test accounts, and unused extensions should be disabled or removed.

Limit permissions

Users should only have access to the features they actually need.

Protect administrative accounts

Administrator accounts should receive additional security controls because they can affect many users.

Use multi-factor authentication when available

MFA can provide an additional layer of protection beyond a password.

Monitor Your VoIP Environment

Security is not something that should only be addressed after an incident.

Regular monitoring can help identify unusual activity early.

Businesses and VoIP providers should consider monitoring:

  • Failed SIP registrations
  • Successful registrations from unusual locations
  • Large increases in outbound calls
  • International calling
  • Calls outside normal business hours
  • Unexpected call forwarding
  • Changes to account settings
  • Multiple simultaneous registrations
  • Unusual traffic patterns

Early detection can significantly reduce the potential impact of an attack.

VoIP Fraud Protection

Fraud protection should be considered a key part of any business VoIP deployment.

Useful controls may include:

Destination restrictions

Limit calling to countries and destinations that the business actually needs.

Calling limits

Set reasonable limits on outbound calling.

Time restrictions

Restrict certain types of calling outside normal business hours when appropriate.

Real-time alerts

Notify administrators when unusual activity is detected.

Automatic blocking

Automatically block suspicious activity based on predefined security rules.

Spending controls

Set limits to reduce the potential financial impact of compromised accounts.

These controls should be configured according to the customer’s actual business requirements.

VoIP Security Best Practices Checklist

Businesses can use the following checklist to improve their VoIP security:

  • Use strong and unique VoIP passwords
  • Change default passwords and PINs
  • Disable unused extensions and accounts
  • Enable MFA where supported
  • Restrict international and premium-rate destinations
  • Monitor outbound calling activity
  • Configure fraud alerts
  • Monitor SIP registrations
  • Use firewall controls
  • Use an SBC where appropriate
  • Use encryption where supported and appropriate
  • Keep phones, computers, and applications updated
  • Secure administrative accounts
  • Restrict access to management interfaces
  • Train employees to recognize phishing and social engineering
  • Review call forwarding and account settings regularly
  • Have a process for immediately disabling compromised accounts

What Should You Do If You Suspect a VoIP Account Has Been Compromised?

If suspicious activity is detected, act quickly.

Depending on the situation, recommended actions may include:

1. Disable or lock the affected account

Prevent further unauthorized access.

2. Change credentials

Reset the affected SIP password, portal password, voicemail PIN, or other credentials.

3. Stop unauthorized calling

Temporarily restrict outbound calling if necessary.

4. Review registrations

Look for unknown devices, IP addresses, or locations.

5. Review call records

Identify when the suspicious activity began and which destinations were contacted.

6. Check account settings

Look for unauthorized call forwarding or other configuration changes.

7. Contact your VoIP provider

If the service is hosted or managed by a provider, report the incident so they can investigate the platform and network side.

8. Review other accounts

If the compromised password was reused elsewhere, those accounts should also be secured.

Is VoIP Less Secure Than Traditional Phone Service?

It is not accurate to simply say that VoIP is less secure than traditional telephone service.

VoIP and traditional telephony use different technologies and therefore have different security considerations.

VoIP is built on IP networking, which means organizations must consider threats such as:

  • Credential attacks
  • Network attacks
  • Malware
  • SIP scanning
  • Toll fraud
  • Internet-based attacks

At the same time, VoIP provides security technologies and controls that can be used to protect communications and manage access.

The important factor is how the VoIP system is designed, configured, monitored, and maintained.

Who Is Responsible for VoIP Security?

VoIP security is often a shared responsibility.

Depending on the deployment, responsibilities may be divided between:

VoIP Provider

The provider may be responsible for:

  • Platform security
  • Infrastructure security
  • SBC protection
  • Network monitoring
  • Fraud detection
  • Platform updates

Reseller or Service Provider

A reseller may be responsible for:

  • Customer configuration
  • Account policies
  • Calling restrictions
  • User provisioning
  • Customer education
  • Monitoring and support

Customer / IT Team

The customer may be responsible for:

  • Internal network security
  • Firewall configuration
  • Wi-Fi security
  • User accounts
  • Endpoint security
  • Employee training
  • Password management

Understanding these responsibilities helps ensure that important security controls are not overlooked.

VoIP Security at UCeasy

UCeasy provides a white-label UCaaS and VoIP platform for telecom providers and resellers.

VoIP security is an important part of delivering reliable communications to business customers.

Security involves multiple layers, including:

  • Authentication
  • Network controls
  • Fraud protection
  • Monitoring
  • Firewall configuration
  • Session Border Controllers
  • Access controls
  • Encryption where supported
  • Proper account management

For UCeasy partners, security should also be considered when provisioning customers, configuring calling permissions, managing user accounts, and troubleshooting unusual calling activity.

If you suspect that an account has been compromised or that fraudulent calling is taking place, the issue should be investigated as quickly as possible.

Quick Summary

VoIP security is about protecting the users, devices, network, platform, and communications that make up a VoIP environment.

The most common threats include:

  • SIP credential theft
  • Toll fraud
  • Brute-force attacks
  • SIP registration hijacking
  • Call interception
  • Voicemail attacks
  • Call forwarding abuse
  • Denial-of-Service attacks
  • Malware
  • Phishing and social engineering

Businesses can significantly improve their security by using strong authentication, network controls, monitoring, encryption where supported, fraud protection, properly configured firewalls, and Session Border Controllers.

Most importantly, VoIP security should be treated as an ongoing process rather than a one-time configuration.

Secure VoIP starts with good architecture, continues with proper configuration, and depends on continuous monitoring.