Intercom calls enable users belonging to the same group to use two phones like an on-door speakerphone. When one user dials a special code before the other user’s phone number, a two-way audio channel is established automatically. The other user does not need to pick up his handset; instead, speaker-phone mode is activated and both users can now talk to each other. Most VoIP phones with the SIP protocol can be used for intercom calls.
Placing an intercom call
- User A dials an intercom prefix, followed by User B’s phone number. His SIP user agent sends an INVITE request to the PortaSIP server (1).
- An authorization request is sent to PortaBilling (2).
- PortaBilling performs several operations:
- Checks that such an account exists and is allowed to use SIP services.
- Checks whether account B belongs to the intercom group under the same customer.
- Checks if the account is registered.
- Based on the results of these operations, PortaBilling sends an authorization response to the PortaSIP server, with a special “auto-answer” trigger (3).
- The PortaSIP server adds the “auto-answer” header to the outgoing INVITE request and sends the call to SIP user agent B (4).
- The two call legs (A and B) are joined together.
- Speakerphone mode is activated immediately on User B’s phone.