ISDN compatibility problem

Background info:
Our company has been using a popular fax server software with pots lines for a few years. The fax server is setup with a (8) line Equinox v.90 multimodum and (2) 3Com Soprtster modems (10) lines total. We recently moved the office and upgraded our communications infrastructure with the "local phone conglomerate’s” ISDN for voice. We now have line cards from the NEC Elite Plus PBX converting the digital signal to analog for the fax servers modems via a distribution block. The problem is we can ONLY receive complete fax transmissions when the receive rate on the fax server software is set to 2400. Any higher we get half, quarter or incomplete faxes. When we setup a POTS line to test we can receive at 14400 without a single problem. Also, when we setup a standard fax machine through the PBX it will work fine. Voice is not an issues. What we have done is made sure we have the latest version of the software on the fax server and modems. My computer vendor is saying the problem is from the PBX . The phone vendor checked environmental issues like lights on the PBX power circuit and the like. We took the db levels on the PBX up and down and checked the PBX settings with the Telco. We also put line attenuators on the cable 2db-8db. My phone vendor is saying it’s a problem with the modems. Telco said test was fine, no problem. Can anyone help shed some light on this? Thanks in advance for any help.
Xland