Cannot Hear The Party That Calls

In January, my parents signed up for phone/internet/cable service with Xfinity.  For four months the system worked great.

Over the past two  months there is an INTERMITTENT problem where they cannot hear the person on  the other end of the call.   This happens if they initiate the call OR if the other person initiates the call.

This is a HUGE problem, as my parents live in an assisted living environment and are not in the best of health (dementia).

It is VERY important that we have functional telephones in their apartment.

I have observed their use of the phones, and there is no obvious error in phone use.  I have restarted the X1 Gateway many times and ensured that the plugs are correctly installed.   Again, this setup, AS IS, worked fine for several months before this problem started.

I have seen other user posts with a similar problem, but I  have yet to see any solution posted.

Please Help.