A SIP proxy is just a component of a voice platform. IP telephony has many services such as call transfer, call pickup, paging, call completion services, and distinctive ringing. A legacy PBX implements all these services in its own firmware and old analog phones were simply dumb terminals. Now, IP phones are intelligent and the SIP server, in most cases, simply relays the requests and replies from endpoints. In other words, to implement the same PBX services that we had in the old analog PBXs, we need full cooperation between phones, proxies, and gateways. As you can see in the following table, there is a straight cooperation between the components to implement a service. It is not possible to implement some services in SIP without support from all the components.
Service |
Proxy |
IP Phone/ATA |
Gateway/SBC |
Media server |
---|---|---|---|---|
Call forward |
Yes |
Some IP phones implement call forward using 3XX redirect messages without any role of the server. |
No role. |
No role |
... |