Land Line Drops Call After 20 Minutes......

When I'm on the phone, my call gets dropped after 20-30 minutes.  We thought it was the phones and we purchased new ones...........not the phones.  Can you please tell me what can be done.  We have been with Xfinity since the start up......  trying to talk with an agent and all he did was try to sell me 'stuff', even after I refused and finally had to just had to hang up....  What can be done about this problem with my phone?  Please.