IP Phones (and SIP clients in general) need to register with a central server mainly because this allows the phone’s location to be known when it is required to receive an incoming call. The “location” is identified by an IP address and port number. In some cases the IP address might be permanent and fixed, but for most IP phones the address can change from day to day.
No matter where your IP phone is physically located, you want it to be able to receive calls. The registration process, which is normally initiated as soon as the phone/PBX is switched on, allows the hardware to register its location in a central server called a Registrar Server. The exchange of messages that enables registration to happen is defined as part of the RFC 3261 (SIP protocol).
To summarise, the IP phone will send a REGISTER request to the Registrar Server. The Registrar Server will ensure that the user credentials sent are correct, then update the Location Server database and send a “200 Ok” response back to the phone. Most IP phones will automatically repeat the registration process every 30 minutes for reasons explained in the next paragraph.