I've never had much of an issue on T1 lines but i've had to play with gains on nearly every POTS line I used. If you have a means to do so you can dial a milliwatt test-tone and see if you are getting good signal through your PBX. This has fixed most CID and Echo issues i've had.

Dont ask me what it is for your area as it can vary and I never remember it anyways.

One of these days i'm going to ask the verizon guy what the number is and write it down on the inside of my bag. It's something like 8988 or 8898 or 8998 or something.