The first thing you need to do is determine if it's in the system or the line. Disconnect the line in question and plug it into a plain old phone to see if it's still bad, if so and you can check it at the DEMARC that's be the next step, but odds are if you disconnect and it's still bad it's a phone company issue. It's either hung up to something or someone or has a short in the line.

Retired phone dude