[VoiceOps] using a T1 to extend PRI service and involuntary fax over sip

[Mark Kent]

I stumbled across a situation where a fax-to-email service gets
inbound calls, from Carrier X, over a PRI into a cisco 5350 which then
relays the calls, via an x-conn PRI, to a directly-attached linux box.
This has worked for years.

About a year ago, it was revealed that the above-mentioned PRI
connects to the edge of Carrier X's SIP infrastructure.  That is,
calls start from some PSTN-connected fax machine, somewhere in the
USA, go through a PSTN-to-SIP gateway, travel as SIP to the building
where this cisco 5350 is, and then go through Carrier X's SIP-to-TDM
equipment for delivery over the PRI to the cisco 5350.

This led to some concern, since we all know that fax over SIP can be
problematic.   But everything was working, hundreds of faxes a day
were pulsing through the system.

Up until September the cisco 5350 was in the same building as Carrier
X's TDM equipment.  In late September, a point-to-point, B8ZS/ESF T1
was used to extend the in-building cross-connect between Carrier X
and the cisco 5350.  The cisco 5350, and related servers, are now
eight miles away (both endpoints in Manhattan, using VerizonBusiness
for the T1, both endpoints "on-net", no ILEC involved).

Since November, maybe half a percent of the faxes fail to work.  
They get a communication error at the start, at the modem negotiation.
The T1 circuit is clean.

Some people think that failures may not have been reported/noticed in
October, but they occured nevertheless.  This would suggest that the
previous set-up was a very delicately balanced system and the moving
of the cisco5350 eight miles away, necessitating the use of a T1 to
carry the PRI to the new location, may be the root cause of the
failures.  Occam's razor reasoning supports this.

The grassy knoll people believe that, in November, Carrier X started
an effort to wring more out of their SIP network. Perhaps they started
using different peers in various parts of the country.  Maybe their
PSTN-to-SIP gateways were tuned to use less bandwidth.  When asked,
Carrier X answered a different question, in a fashion similar to a
politician.

I'm wondering whether any experts here have an opinion to offer?

Thanks,
-mark