Highly Unusual Problem ... Need Help

Unanswered Question
Feb 5th, 2008
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UCM 5.1(2) Cluster with PRI to PSTN and T1-CAS trunk to branch office PBX.


A client reported the following incident...


IP Phone Caller A makes a call to PSTN Phone X. The call progresses and completes normally.


Branch Office PBX Phone calls IP Phone A across T1-CAS trunk. Call appears to proceed and terminate normally


PSTN Phone X calls back IP Phone A and tells them they received a call, answered and heard IP Phone A's side of the conversation to the branch office phone.


I'm puzzled ... two separate circuits, two separate calls. No other reports of the same issue and can't reproduce it.


Any ideas would be GREATLY appreciated.

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Tommer Catlin Tue, 02/05/2008 - 09:12
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Sounds fishy to me. It really would not be possible to hear other conversations. The correcting routing would be the following:


PSTN calls comes into a MGCP controlled router.

MGCP router picks up the call, routes to CallManager.

CallManager analyzes the DID and does a lookup.

DID matches a translation pattern or route pattern that pushes it the CAS T1 trunk at MGCP router.

MGCP Router routes call from callmanager through T1 Trunk to PBX

The call is then answered on the PBX side.

CallManager drops out of the conversation. Now it is just MGCP router

PBX analyzes DID and routes to phone device.


If you think there is spoofing of packets going on, it would be at the Router. Possibly the T1 Trunk is not tearing down the call completely, but that seems odd.

- Out of sync with the PBX and MGCP router

- Channels are "flapping" on the T1 Port or dropped packets


Its hard to say with the a remote PBX. Every PBX is not quite standard when it comes to their "T1" configurations. Is it QSIG controlled? Diversion packets coming and going correctly?


But your scenario sounds like fishy. Someone reported it, but it probably can never be reproduced. (at least from what I am reading or technically know about CAS and Cisco)



Brian Carscadden Tue, 02/05/2008 - 11:23
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I received some additional details from a tech on-site. Not much, but it helps clarify the call flow. This is different from our symtoms earlier, but just as odd. A call from an IP Phone @ x285 dials 133 which is a route pattern that points to a T1-CAS circuit to a a branch office PBX, but apparently ALSO grabs a T1-PRI circuit and dials a PSTN number and plays the outgoing audio stream from x285.

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