I suspect it is not a ring voltage problem, but certainly check with an analog VOM set on AC and see what you have for ring voltage.
I have encountered modem problems when the PBX was sending a double ring burst. Sometimes the modem would count both rings as one, sometimes two seperate rings, sometimes the rings were just too short for the modem to consider them a valid ring.
Any chance you can enable verbose settings in the modem and run a terminal program while ringing the port? If you can, you should see a ring indication every time the modem line rings IF the modem is actually seeing a ring.
Only other obscure thing I can remember is a similar problem with off premise cabling. We had a 200 pair between buildings, protectors on both ends. The customer had requested an analog port instead of a digital one. We moved the jumper, it worked but wouldnt ring. After some tracing we found the modules in the protector on that pair were clamping voltage at 70 volts or so. They were the red ones used for digital sets. We replaced them with the standard black modules that allowed full ring voltage to pass and it was fine. Obscure situation but it happens.
Good Luck
Daron J. Wilson
Telecom Manager
LH Morris Electric, Inc.
daron.wilson@lhmorris.com