Are these real PRI's delivered on a T-1, or Voip PRI's being delivered on an HDSL or similar 1 pair circuit?

I have several 8300's and 8100's using faxes off of analog ext's and don't have any issues with "real" PRI's.

If these are coming in on HDSL, sometimes the carrier can setup specific DID numbers to be uncompressed on an inbound call to that specific DID, then you have to have a few channels dedicated for outbound for faxes that are uncompressed. That way when a fax dials out it always hits that uncompressed group of channels.

PITA, but it works.
D