What is a SIP Proxy or SIP Registrar?

One of the great benefits of VoIP telephony is the ability to receive calls at multiple locations, whether simultaneously, sequentially, or at multiple locations depending on the time of day. This is made possible by combined efforts of a SIP Proxy and a SIP Registrar, which can run on dedicated hardware or as services on a PBX.

The Promise of the SIP Address

SIP geeks seem always to be dreaming of a day when everyone’s business cards include the following:

SIP/Email: john@company.com

In order to initiate a call to john@company.com, this address has to be translated into the actual IP address of Johns phone. In order to get this, INVITE packet is sent to the SIP Proxy server at company.com. The SIP Proxy will then do a DNS lookup at company.com’s SIP Registrar. These services often run on the same host. The SIP Proxy then returns the IP address of John’s phone, and the caller can send another INVITE packet directly to John’s phone.

Dynamic Call Routing

By changing the entries at his SIP Registrar, John can manage where and when his calls are directed. In the evenings, he can have his calls routed to his cell phone. During his meetings, he can direct the SIP Registrar to return the IP address of his voice mail.