A SIP device can register with a service, e.g. with the FB2500, or with a SIP carrier. This is like logging in and means that incoming calls are then sent to the device. The device will renew the registration periodically to stay logged in, and if it fails to do this then incoming calls will fail.

This process uses a username and password for security. Obviously you also have to say where to register, the proxy specifies IP address or host name of the SIP service with which you are registering, and the registrar defines the hostname to use in the registration.

This process works well if the service does not have a fixed idea of where you are, which is normally the case for SIP handsets. Even on a local network the IP of the handset will normally be dynamically allocated with DHCP, and for a SIP carrier the IP could be anywhere in the world.

To make an outgoing call via a SIP carrier you have to send the call details to a proxy. In the case of the FB2500 acting as the carrier, the same address is used for registrar and proxy.

The process uses a username and password in much the same way as registration, and they are usually the same details. This checks that the device is allowed to make the calls, and allows the right person to be billed for the call.