In the past we’ve covered what a SIP Registrar does, how to build one, and covered some misconceptions about what being Registered means, but there’s a few little-utilized features of SIP Registration that are quite useful.

A lot of people think there’s a one-to-one relationship between a registration Address on Record, and a username.

That doesn’t have to be the case, there are some platforms that only allow a single registration for a single username, but the RFC itself allows multiple registrations for a single username.

REGISTER requests add, remove, and query bindings.

A REGISTER request can add a new binding between an address-of-record and one or more contact addresses.

Registration on behalf of a particular address-of-record can be performed by a suitably authorized third party.

A client can also remove previous bindings or query to determine which bindings are currently in place for an address-of-record.

RFC 3261 – 10.2

Let’s say you’ve got a SIP phone on your desk at the office and at home.

What we could do is create a different username and password for home & work, and then setup some time based forward rules to ring the office from 9-5 and home outside of that.

You could register both with the same username and password, and then unplug the one at home before you leave to work, get to work, plug in your office phone, unplug it before you leave to go home, and when you get home plug back in your home phone, or if multi-device registration is supported, register both and have incoming calls ring on both.

Admittedly, platforms that support this are the exception, not the rule, but the RFC does allow it.

The other little known feature in SIP Registration is that you can query the SIP Registrar to get the list of Addresses on Record.

So there you go, factoids about SIP REGISTER method!