Quote Originally Posted by VOIPoDanielC View Post
If you have more than 4 phones connected to the same port on the ATA, you may be running into a voltage or current problem.

Does it happen to be the phone furthest from the ATA when you have multiple phones connected?
It is also possible that he is distributing the line using house wiring that is still connected somewhere to his old provider. Sometimes old wiring will be connected in odd places and current will backfeed into the line, causing the ringer to get a spike whenever the line discharges.