I have an issue with my phone service that is perplexing and results in four support organizations pointing the finger at each other.

I have an Avaya system with a five line hunt group coming in to the system and six telephones. Initially this system would drop calls. The system was replaced and the dropped calls have subsided. Now, I sometimes get a single ring and a flash on one of the lines, then there's no further indication that a caller is there. If I pick up the line below the in-use line, the caller is sometimes there. This happens to about 1 out of 20 calls. I've eliminated anything internal (I think). We suspected it may be low ringing voltage, but Sprint indicated that was good. It was suggested (though not proved) that all the affected calls may be coming in on the 800 number. This put the ball in AT&Ts court, though they indicated there was nothing that they had that could cause those symptoms.

Any suggestions as to what could cause this behavior?

Thanks