LightSpan
Might be a long shot...Had a customer once that was complaining they couldn't receive faxes and were served off of LightSpan, they were also the only customer with this trouble. Did the normal tests for leakage and noise and all was fine. After a few trips out to their location we checked to see what the ring voltage was. The first ring was the normal 90Vac but then it dropped to like 50 Vac, and just kept getting lower and lower until the fax modem wouldn't even recognize that there was a call coming in. I can't remember if it was a bad power board or ring generator but you could easily put a Volt/Ohm meter across tip and ring at the d-mark and see if you are getting a constant 90Vac.
