A VoIP carrier is a service provider that can accept outgoing calls, and route incoming calls. Typically a VoIP carrier is expecting a handset to register with the carrier, and will then send calls to the registered device. It is also possible for a VoIP carrier to send calls to the FB2500 using a fixed pre-set configuration.

To set up a VoIP carrier where the FB2500 registers with the carrier you need to specify the registrar attribute. This can be a host name or IP address. You also need to specify the username and password. For incoming calls you need to specify the extn that is logically dialled when a call comes in from this carrier - this can be the extension number of a telephone or hunt group.

To set up a VoIP carrier for outgoing calls you need to specify the proxy. This can be a host name or IP address. You also need to specify the username and password.

For a carrier that sends calls to the FB2500 without registration, you will need to set the to attribute to either the full address used in the To: of the incoming connections, or at least the @domain part. If calls can come in to multiple numbers, you also need to set the incoming-format so that calls can be routed. In this case the username and password can be specified and are used to validate the credentials of the incoming call.