Call Forwarding not always working

Hi,

I'm an IT consultant that works with Comcast a lot but I've run into a strange problem.  I have a customer with two lines in a hunt group.  He forwards the main number to his cell every evening.  Its worked like this for 4 years.  Starting three weeks ago his cell would ring and he would get the caller id info but when he answers he hears nothing.  On the caller side the call stops ringing and then they hear nothing.  This happens probably 75% of the time now and he gets a lot of calls after hours.

First we though it was a cell phone issue (Verizon is the carrier).  I went through several stages of their tech support and he even switched phones but that did not help.  Forwarding calls to two other cell phones including mine gives the same result (one cell phone is Sprint).

I have forwarded his second line and it works perfectly everytime we test it.

So I've called Comcast business support three times to investigate the issue but they find nothing wrong.  I know the local techs so they created a ticket and came out and replaced the MTA (I believe that is the right name for it) but the problem persisted.  I also replaced the phones in his office just to make sure but the problem still exist.  I'm not sure where else to turn.  At this point he is considering just switching back to the phone company but I dread the super slow internet speeds he will have compared to what he is used to.

Can someone in the forums help???