McDewey

In a SIP environment, the endpoint that is hosting a three-way call can drop out and arrange to have the remaining two parties connected together. With SIP, the system accomplishes this by using REFER with embedded replaces. Prior to this action, two calls with four dialogs exist: 1. A.1 to B call: a. A.1 to Unified CM dialog. b. Unified CM to B dialog. 2. A.2 to C call: a. A.2 to Unified CM dialog. b. Unified CM to C dialog. Phone A can drop out of the call by sending an in-dialog REFER on dialog A.1 with an embedded replaces header that specifies dialog A.2. Unified CM invokes its attended transfer feature, which results in the remaining parties being connected together. Refer to Attended Transfer, on page 28 for details regarding the operation of that feature. Call Forwarding 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. Unified CM supports three types of forwarding: • Call Forward All (also known as Call Forward Unconditional) • Call Forward No Answer • Call Forward Busy Only, in call forward no answer case, does the call actually get presented to the original called party. 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 Unified CM, so 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 must use 302 (see Endpoint Returns 302 Redirect, on page 31) and 486 (see Endpoint Returns 486 Busy, on page 32 response codes, respectively, to the INVITE. Unified CM informs the calling party that their call has been forwarded via “Remote-Party-ID:” headers in updated 180 messages. This type of forwarding does not get communicated to the calling party. For example: Remote-Party-ID: "Line 1030 Name" sip:1030@172.18.203.78;party=called;id-type=subscriber;privacy=off;screen=yes Unified CM indicates forwarding to the called (or current forwarded-to) party by using “Diversion:” headers in subsequent INVITEs. 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 SIP Line Messaging Guide (Standard Edition) for Cisco Unified Communications Manager 30 SIP Standard Line Interface Call Forwarding