Answering side hears echo from cell phone call

[pdvmike]

We have an ISDN PRI to a Cisco 2833 routing calls to
a SIP application server. Sometimes (1/50) when a call
from a cell phone is answered we find that the greeting
message is digitaly looped back, the caller hears the
message but the application server only gets the greeting
message looped back. The caller's voice path does not cut
through. We have tracked this out to the PRI by monitoring
the bearer channel so it is not a problem with the Cisco
or the SIP AS. This never happens with a normal PSTN call,
only Cell calls. Any ideas or suggestions?
Thanks

 

[ISDNman]

Your provider has a trunk group somewhere that is in loopback mode.

Here is a paper I wrote based on empirical experience in solving similar issues:

http://www.telos-systems.com/techtalk/PSTN/default.htm

You will need to be persistent.