Calls drop after 15 minutes

Our phones drop out after about 15 minutes on a call. We can hear the caller on the other line, but they can not hear us. They disconnect and try to call back, when we pick up their call they still can not hear us, and it engages our 3rd roaming line temporarily (2 lines at once). After about 30 seconds the phones seem to reset and we can accept calls again...We're using Comcast Business internet and phone with AT&T 1070 4-line phones