one line has dial tone and can call out but when called it rings once and drops. caller hears one ring and then nothing. att tested line and its fine coming in, but said they detected a short on the line. thought it could be with an extension and suggested i remove all, then test one by one. sound right?

Swap the modular plugs, if line 1 has problem swap 1 with 2 and see if problem goes to line 2. If it does them problem is in line or wiring somewhere up to the plug. If it stays on line 1 the line port is bad.