Secure your VoIP phone system against toll fraud, eavesdropping, and SIP attacks — configuration hardening, encryption, and monitoring best practices.
VoIP phone systems are a frequent target for cybercriminals — toll fraud alone costs UK businesses millions annually. This security guide covers the essential steps to protect your business phone system from the most common attack vectors. From SIP trunk hardening and SRTP encryption to toll fraud prevention and call monitoring, it provides practical, actionable security measures that any IT team can implement to keep business communications secure.
IT managers, security teams, and business owners who need to secure their VoIP phone system against toll fraud, eavesdropping, and other telephony-specific cyber threats.
Toll fraud happens when attackers gain access to a phone system, often through weak SIP credentials, and route expensive international or premium-rate calls through it, sometimes racking up thousands of pounds in charges within hours. UK businesses without call barring or usage alerts in place are particularly exposed, often discovering it only after receiving the bill.
Use strong, unique credentials for SIP registration, restrict access using IP allowlisting where possible, enable TLS encryption for signalling, and configure a session border controller or firewall specifically for SIP traffic. Disabling international calling by default and requiring explicit authorisation reduces exposure to toll fraud significantly.
SRTP, or Secure Real-time Transport Protocol, encrypts the actual voice media of a call, preventing eavesdropping on conversations travelling across the network. Without it, VoIP calls can potentially be intercepted on an unsecured network, making SRTP a baseline expectation for any business handling confidential conversations.
Yes, this guide includes a section on setting up monitoring and alerting to detect unusual calling patterns, such as sudden spikes in international calls, which is typically the earliest warning sign of a compromised phone system.
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