What do you mean by "audio cutback". Do you mean drop outs, low level, distortion? Is this intermittent, or on each and every call, for all users?
Do you use a digital receptionist? Does that sound OK? How does the outgoing message sound?
Have you tried emailing messages to the users, how do they sound when played back elsewhere? In other words, are the recordings themselves good and it's playback that is the issue.
Did this just start, or has it always been this way?