SIP trunking is becoming a very popular way to connect to the public switched telephone network (PSTN). Session Initiation Protocol (SIP) is a signaling protocol, widely used to set up and end multimedia communication sessions, such as voice and video calls over the Internet. Instead of a line from your local provider (AT&T, Verizon, etc.), you connect to one of several SIP trunk providers, sometimes called an Internet Telephony Service Provider (ITSP). They can provide phone numbers in most area codes and provide features such as Direct Inward Dial (DID) and Caller ID over the SIP trunk. Think Vonage, but on an enterprise scale.

What is confusing to many people is that SIP Trunk is a logical connection. It is not like POTs, T1s or PRIs which are physical connections. The most common ways to connect to the ITSP are through the Internet or a private MPLS circuit. The biggest limitation of Internet use is the lack of quality of service (QOS), but it is surprising how well it works.

SIP trunks have been supported by Cisco Unified Communications Manager for a while. Configuring SIP trunks directly in UCM has its limitations. This is why Cisco has recently added extended SIP trunking support on its routers. Cisco Unified Border Element (CUBE) is a set of features that can be added to the voice gateway. There is an additional cost for the software, but when compared to the cost savings of going SIP, it can make a lot of sense.

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