This section describes several methods for implementing call transfer across VoIP networks:
? H.450 and SIP
? Hairpin Routing
? Empty Capabilities Set
H.450 and SIP
The ITU-T standards-based H.450.2 transfer method and the Cisco method operate in a similar way. In both cases, when a call transfer occurs, a control message is sent back to the transferee party to request that the transferee initiates a follow-on call from the transferee to the final transfer-to destination. In the H.450.2 case, the follow-on call originated by the transferee can act to replace the transfer consultation call that is in progress between the transferor and the transfer-to destination party. The consultation call between transferor and transfer-to and the original transferee-transferor call are not torn down until the "replaces" operation is completed successfully. The term replaces is used here in the context of "Call 2 replaces call 1." If for any reason the replacement operation fails, it is usually possible for the transferor to reconnect the call to the transferee. The H.450.2 mechanism works in a manner similar to the REFER method used for SIP VoIP calls. The Cisco transfer mechanism does not support the call replacement mechanism and, therefore, allows you to perform only blind call transfers. This proprietary method is similar to the older BYE/ALSO method that was used to perform blind transfers for SIP VoIP calls. The BYE/ALSO method has been mostly superseded by the SIP REFER method.
Both of these H.323 call transfer methods result in an optimal direct call path between the transferee and the transfer-to party after the call transfer is committed.
The third alternative is to hairpin route the VoIP call transfer. In this case, the original transferee-to-transferor VoIP call leg is kept, and a second transferor to transfer-to VoIP call leg is created for the consultation call phase of the transfer.
For the sake of completeness, it is worth mentioning a fourth alternative for call transfers: Empty Capabilities Set (ECS). Cisco Unified CME does not support the instigation of transfer using ECS.
Cisco Unified CME VoIP Call Transfer Options
Your Cisco Unified CME system by default is set up to allow local transfers between IP phones only. It uses the Cisco H.323 call transfer extensions to transfer calls that include an H.323 VoIP participant.
To configure your Cisco Unified CME system to use H.450.2 transfers (this is recommended), set transfer-system full-consult under the telephony-service command mode. You also have to use this configuration for SIP VoIP transfers.
To enable hairpin routing of VoIP calls that cannot be transferred (or forwarded) using H.450, use the allow-connections command. The following example shows a call transfer configuration using this command.
voice service voip
allow-connections h323 to h323
The configuration shown in the preceding example turns on the H.450.2 (transfer-system full-consult) and H.450.12 services, allows VoIP-to-VoIP hairpin call routing (allow-connections) for calls that don't support H.450, and permits transfers to all possible destinations (transfer-pattern). The transfer permission is set to .T to provide full wildcard matching for any number of digits. (The T stands for terminating the transfer destination digit entry with a timeout.)
The following example shows a configuration for more restrictive transfer permissions.
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