land line and cell phone when connected to xfinity wifi dropping calls

My land line (with Xfinity) for the past 6 months is dropping calls after about 4 minutes.  I switched to just using my cell phone......Now, my cell phone is doing the same thing (only when I am home and the phone is connected to my xfinity internet).  I have purchased new land line phones (3, and all do the same thing, so not a plug in issue), and got the same results of dropped calls.  I tested my cell phone when not home and not connected to xfinity wifi and it doesn't drop calls.  I have restarted the router.  So I have in both cases of land line and cell phone eliminated that it is the actual phone.  Nothing is working, yesterday I was on my cell phone and the person had to call me back 8 times within 20 minutes.  So now I don't really have any phone service.  I need help!!