Phone Cutting In and Out: Can't understand the person I'm talking to

We switched to Cox phone to bundle on July 23, 2016.  Three times they've been out to see why we get shorts in the phone while talking.  It mostly happens during incoming calls but once I called cox and it was so bad I had to give my cell number for them to call back.  Sometimes it cuts out for a word, but sometimes full sentences.  First visit they fooled around in the box and blamed Verizon.  Second visit...same.  Third visit seemed promising.  The tech put a new hard wire line to a jack claiming the traditional jack can get dust on it.  So, hard line from modem to wall.  That lasted longer but now it's happening again.  It's probably a voltage problem but how do they find the issue.  These guys are guessing.  One thing is that there are two jacks in the house that were on a second line that was disconnected when switching from verizon to cox.  The jacks used to be on the main line so I'm assuming it would be easy to activate them on our main line.  I'm thinking that could be the problem.  Any ideas..Actually, this is my parents' house.  They're 87 years old and very frustrated.  Thanks in advance.