trico, I had thta problem with ACD agent phones. It was the firmware in the phone causing it to just pop instead of ringing. If you reset switch are phone it would work for next call, but would start poping again after that.

PVJ, the probem I had with Telco was that the lines going through there slick where not sending more than 52 volts for tring voltage. Switch would shoe it ringing in the "C" screen, but not ring the phone. Droped the ringing threshhold and phone would ring, but other lines started doing strange things. We found 1 card at slick that controled 4 lines that was bad, replaced and all was well.

If all else fails, use a BFH.