/mcpTransfer SIP line-side Transfer uses the REFER message, and REFER with an embedded Replaces header, as per RFC 3515. The following three participants exist for call transfer: • Transferee—The person who is being transferred. • Transferor—The person who is transferring the call. • Transfer Target (Target)—The person who is receiving the transfer. Unified CM supports the following three types of transfer: Attended Transfer With attended transfer, the transferor places the transferee on hold and calls the target. After conversing with the target, the transferor completes the transfer and drops out of the call. The transferee automatically gets taken off hold and connected to the target. Attended transfer involves two somewhat independent dialogs at the transferor device up until the time the device sends a REFER with embedded replaces header. When this message is received, Unified CM knows that the calls are associated. Because Unified CM is a B2BUA, a REFER with embedded replaces does not trigger an INVITE with replaces from the transferee to the transfer target. The dialogs between Unified CM and each phone stay independent. Instead, Unified CM reINVITEs (and UPDATEs) the transferee and transfer target to connect them together. During this process, the transferor will receive sipfrag NOTIFY messages. After the connection is complete, both dialogs between Unified CM and transferor get BYE’d. The following more detailed view shows what happens when the REFER is received: 1. Split transferor and transferee call: • reINVITE to disconnect media. 2. Split transferor and transfer target call: • reINVITE to disconnect media. 3. Join transferee and transfer target call legs: a. reINVITE to connect media. b. UPDATE display name and number via the Remote-Party-ID header. 4. Clear transferor dialogs. Early Attended Transfer With an early attended transfer, the transferor places the original call on hold and calls the target. Upon receiving a ringback tone, the transferor transfers the call to the target and drops out of both calls. The transferee receives a ringback while the target phone is alerting. When the target answers, the system establishes a connection between transferee and target. SIP Line Messaging Guide (Standard Edition) for Cisco Unified Communications Manager 28 SIP Standard Line Interface Transfer