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 configure your Cisco Unified CME system to permit transfers to nonlocal destinations (VoIP or
PSTN), set the transfer-pattern command under telephony-service. The transfer-pattern command
also allows you to specify that specific transfer-to destinations should receive only blind transfers. You
also have to use this configuration for SIP VoIP transfers. The transfer-pattern command allows you to
restrict trunk-to-trunk transfers to prevent incoming PSTN calls from being transferred back out to the
PSTN (employee toll fraud). Trunk-to-trunk transfers are disabled by default, because the default is to
allow only local extension-to-extension transfers.
To allow the H.450.12 service to automatically detect the H.450.2 capabilities of endpoints in your
H.323 VoIP network, use the supplementary-services command in voice service voip command mode.
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
supplementary-service h450.12
allow-connections h323 to h323
telephony-service
transfer-system full-consult
transfer-pattern .T
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.
telephony-service
transfer-system full-consult
transfer-pattern 1...
transfer-pattern 2... blind
This example permits transfers using full consultation to nonlocal extensions in the range 1000 to 1999.
It also permits blind transfers to nonlocal extensions in the range 2000 to 2999.
Notes Regarding H.450.12 and ECS
H.450.12
You can compromise between the H.450.2 and hairpin routing call methods by turning on the H.450.12
protocol on your Cisco Unified CME system (this is recommended). You must be using at least
Cisco Unified CME 3.1 to use H.450.12. With H.450.12 enabled, your Cisco Unified CME system can
use the H.450.12 protocol to automatically discover the H.450.x capabilities of VoIP endpoints within
your VoIP network. When H.450.12 is enabled, the Cisco Unified CME system can automatically detect
when an H.450.2 transfer is possible. When it isn’t possible, the Cisco Unified CME system can fall back
to using VoIP hairpin routing. Cisco Unified CME also can automatically detect a call from a
(non-H.450-capable) Cisco Unified CallManager.
Empty Capabilities Set
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. But
because a Cisco Unified CME router also has the full capabilities of the Cisco IOS Release H.323 voice
infrastructure software, it can process receipt of an ECS request coming from a far-end VoIP device. In
other words, a Cisco Unified CME system can be a transferee or transfer-to party in an ECS-based
transfer. A Cisco Unified CME system does not originate a transfer request using ECS. The problem with
ECS-based transfers is that in many ways they represent a combination of the worst aspects of the
end-to-end dependencies of H.450.2 together with the cumulative problems of hairpin for multiple
transfers. Many ECS-based transfer implementations do not allow you to transfer a call that has already
been transferred in the general case of VoIP intersystem transfers.
Disclaimer : The Extract is from Cisco Systems Documentation
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