" We have had the following equipment hooked to our phone line since January:
1)An old black dial phone with gong ringer (can't be turned off)
2)A 14.4 K internal modem (don't know mfg)
3)A Unisonic phone with ringer equiv. of 0.6A, but with ringer turned off. Up until three weeks ago we had no problems. The black phone rang fine in the morning. When I got home from work in the evening, the ring was strange--weak and tinkly, as if it weren't getting enough power. Other symptoms: The ringer on the black phone now "taps" the gong both when the modem dials and disconnects, when the Unisonic is picked up off the cradle, and when we dial out"
This sounds like a problem with the service. It sounds like the junction/relay that the phone company uses (NOT the interface box at your house) is sending out higher than normal voltage whenever the other equipment changes the on/off hook state the line. A voltage spike that is close enough to ring voltage to cause the solinoid inside the old phone to briefly activate and tap the bell
On Hook: 40 to 50 VDC V---^---Spike during switching phase* Off Hook: 4 to 6 VDC
Ringing: 90 to 100 VAC
Do any of the phones ring normally when you get a call?
*A DC spike would cause the mechanical solinoid to move only once, as it needs the AC during the ringing phase to occillate (continuously ring) the old phone.