AT&T Phones Can't Connect to Comcast Business line - intermittently

Hi!

I had an issue earlier today that has happened in the past.  The problem has been fixed, but not before it cost us at least one phone to go order.  AT&T land and mobile based phones, from time to time, can't connect to my Comcast business line at my restaurant.  As stated before, the problem is gone and the phones can again connect, but being this is neither the first, second, nor 3rd time this has occured, I am looking for help!

Is there anything I can do or Comcast can do for me to prevent this?  I am on a verizon phone, so I don't have a way of testing this myself.

I have also been on hold for over 20 minutes now with Comcast support when I asked if we could escalate this concern.  Comcast services here at the restaurant have been great, but this AT&T issue (now made worse by this exceedingly long hold time - can I submit a review on the person who took my call? can't remember his name) is a problem I very much want to correct again.

So again, please help!