18.2. Registration and Proxies

One of the common confusions with SIP/VoIP is the way registrations work.

18.2.1. Registrar

A SIP device can register with a service, e.g. with the FB2900, 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.

Note

It is possible to have a VoIP carrier that does have a pre-set idea of where calls are to be sent, and sends the calls without registration. In this case there is no registration process but the handset/device has to be able to accept calls from the carrier. Again, a username and password are used for security, but this time it is the device checking the credentials of the carrier.

18.2.2. Proxy

To make an outgoing call via a SIP carrier you have to send the call details to a proxy. In the case of the FB2900 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.

Tip

You can have a case of incoming calls working and not outgoing, which means registration has worked but somehow you have incorrect proxy details. The other way around, where outgoing calls work and incoming do not would mean the registration is not working, but the proxy details are correct. The logging options can be very useful to help diagnose problems.