[cisco-voip] Anti-Tromboning Tie Lines to PBX

[Anthony Holloway]

I'm not sure how often this gets configured out in the wild, but I do know
that there are few different flavors of this feature, depending on your
scenario.  E.g., CUBE Media Anti-Tromboning
<http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/configuration/cube-book/media-path.html#concept_674530029296875008039300537109374>

The one I'm interested in would involve a tie line to a legacy PBX, and I'm
not married to the IP technology: SIP, H323, MGCP, whichever will get the
job done.

CUCM ---IP---> VGW ---T1/PRI---> PBX

The call scenario I'd like to see if we can avoid is when a call is
established from CUCM to the other PBX, and then a phone on the far side
transfers the call to an IP Phone on the CUCM side, I'd like the PRI usage
to drop to 0 trunks in use, instead of nailing up 2 trunks for the duration
of that call.

I'm looking for your experience and feedback in configuring anti-tromboning
in this scenario.  One of the hardest parts in researching this, is knowing
what terms to search for.  I think I've narrowed down the terminology to
QSIG Path Replacement, as described in the CUCM SRND
<http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/8_5_1/ccmsys/accm-851-cm/a08procl.html#wp1156114>
.

Though, without real-world working experience, it's hard to know if I'm
right or not.

So, I know there's some good legacy telco knowledge out on this list.  What
do you know?  Thanks!
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