Landline outgoing call are garbled

This has been going on for a while. I can hear calls, but the caller either cannot hear me or it sounds very distorted. So far I've had 2 Cox techs out to my home. They replaced the phone modem, tested the internal wiring and external connections and it still does not work. I plugged in a known working phone directly into the modem and it still doesn't work. The Cox tech plugged his test set directly into the modem with the same results. I need my home phone. I'm getting ready to drop Cox completely. I need something reliable.