Call Forwarding occurs when a call does not get answered by the original called party but, instead, gets presented to one or more subsequent forwarded parties. Cisco Unified CM supports three types of forwarding:
In only in the call forward no answer case does the call actually get presented to the original called party. Cisco Unified CM detects call forward all and call forward busy prior to sending an INVITE to the called party, so forwarding bypasses that party. Call forward no answer will get detected via a timer in Cisco Unified CM, so Cisco Unified CM will initiate the canceling of the call to the original called party.
Older Cisco phones that use SIP or third-party SIP phones may elect to implement forward all and forward busy locally on the phone, in which case they will need to use 302 (see Endpoint Returns 302 Redirect) and 486 (see Endpoint Returns 486 Busy response codes, respectively, to the INVITE.
Cisco Unified CM informs the calling party that their call has been forwarded via “Remote-Party-ID:” headers in updated 180 messages. The type of forwarding does not get communicated to the calling party.
Remote-Party-ID: "Line 1030 Name"
Cisco Unified CM indicates forwarding to the called (or current forwarded-to) party by using “Diversion:” headers in subsequent INVITEs. Cisco Unified CM will report, at most, two diversion headers. The first will indicate the last forwarding party, and the second will indicate the original called party. In a single-hop forwarding case, the system uses only a single diversion header because the original called party and last forwarding parties are the same. In a three-or-more-hop case, the intermediate parties do not get communicated to the current forwarded-to party. For example
Diversion: "Line 1020 Name"
Diversion: "Line 2020 Name"
Diversion: "Line 3020 Name"
The phone may activate Call Forward All via a softkey.