Security is a consideration that businesses sometimes overlook when implementing VoIP systems, focusing on call quality and features while treating the communication channel itself as an afterthought. The reality is that unencrypted VoIP calls are technically interceptable on the same network, and the business information exchanged in phone calls can be as sensitive as anything transmitted through email or file sharing systems. Understanding SRTP and the broader security landscape for VoIP helps businesses make informed decisions about how their communications are protected.
Traditional landline calls travel over a dedicated physical circuit that is difficult to intercept without physical access to the infrastructure. The security model was inherently physical. VoIP calls, by contrast, travel as data packets across shared IP networks, including the public internet. Like all IP traffic, these packets can theoretically be captured at any point along the network path between the caller and the called party.
Without encryption, captured VoIP packets can be reassembled into an audio recording of the call. This is not a theoretical exercise. Tools exist that can capture and reconstruct unencrypted RTP audio streams from network traffic in a relatively straightforward way. For businesses discussing sensitive client information, proprietary business data, or any confidential matter on phone calls, transmitting those calls as unencrypted audio over the internet represents a genuine information security risk.
SRTP stands for Secure Real-time Transport Protocol. It is an extension of the RTP protocol, which carries the actual audio data in VoIP calls, that adds encryption, message authentication, and replay protection to the audio stream. SRTP uses AES encryption to scramble the audio content of the call, making captured packets unreadable without the encryption keys that were established when the call was set up.
The encryption keys used by SRTP are typically negotiated during the call setup process using DTLS, which stands for Datagram Transport Layer Security, or through SDES, which stands for Security Descriptions. DTLS is the more secure approach, as it negotiates keys over an encrypted signaling channel. Most modern VoIP platforms support SRTP with DTLS for media encryption.
SRTP encrypts the audio media stream, but a complete VoIP security implementation also needs to protect the SIP signaling that controls the call. SIP signaling carries information including the caller’s identity, the called number, call routing information, and authentication credentials. Without encryption, SIP messages sent over the internet are readable in the same way that HTTP web traffic was readable before HTTPS became standard.
TLS, or Transport Layer Security, encrypts the SIP signaling channel in the same way that HTTPS encrypts web traffic. The combination of TLS for SIP signaling and SRTP for media encryption provides end-to-end protection for both the control and content of VoIP calls. This combination is sometimes referred to as SIPS plus SRTP, and it represents the current best practice for secure VoIP communication.
Beyond call encryption, VoIP security encompasses several additional areas that businesses should address. Toll fraud is one of the most financially damaging VoIP security incidents. Attackers who gain unauthorized access to a VoIP system can make large volumes of international calls, generating enormous charges before the abuse is detected. Strong authentication for SIP accounts, including complex passwords and where possible IP-based access restrictions, significantly reduces the risk of unauthorized account access.
Denial of service attacks targeted at VoIP infrastructure can disrupt call service by overwhelming the SIP server with illegitimate traffic. A Session Border Controller, as described elsewhere, provides a layer of protection against these attacks at the network boundary. For hosted VoIP services, the provider typically implements DDoS protection at the infrastructure level, but businesses should confirm this as part of their provider evaluation.
Physical security of the phones themselves is an often-overlooked element. Phones left unattended in accessible areas can be used to make unauthorized calls or to intercept conversations in the vicinity. Default passwords on phones should always be changed during deployment, as many automated attacks specifically target VoIP devices with factory default credentials.
When evaluating hosted VoIP providers from a security perspective, several questions are worth asking. Does the platform support SRTP and TLS for call encryption by default, or is encryption an optional feature that must be enabled? How are authentication credentials stored and managed? What protections exist against toll fraud, and how quickly are anomalous call patterns detected and acted on? What certifications or compliance standards does the provider maintain that are relevant to your industry?
For businesses in regulated industries including healthcare, finance, and legal services, these questions are not optional due diligence items. They are requirements that the chosen VoIP provider must satisfy before being considered appropriate for handling professional communications. A provider who takes VoIP security seriously will have clear, specific answers to all of these questions and documentation to support them.
