Customer called that his desk phone goes dead when on a call.The carrier is comcast.
Went in could not duplicate any trouble. He called 2 days later.
While i was there a incoming call went dead, he handed the hand set to me, there was silence
but the lcd screen was on with CID info and the co line button in use was green. In a few seconds
the party came back on, he wondered was happened he was talking but no response. The customer
then said they lose comcast internet almost every week. I think comcast is the trouble and told him to
call comcast to report this problem and keep a record if this continues.
Thank you Gary s.

I've seen this quite frequently with Cox and Spectrum services using Ubee modems. They do all kinds of strange things when they go bad. One at my parents' house would actually answer with a modem tone periodically. The only thing you can do is beg Comcast to replace their modem. Your description of the display and line key status pretty much proves that the line is going dead outside of the system.

One other thing to double check: Many of the modems have an RJ14 for lines 1/2 on the first jack and RJ11 for line 2 only on the second one. Same for lines 3/4 on the third jack. Make sure that there's nothing connected to the second pair(s) of the connections to jacks 1 and 3 on the modem. I know it's a stretch, but you need to exhaust all of your resources.