I have an S7 and my wife has an iPhone 7 Plus. Both of our phones are in excellent condition without any issues/problems. When we call each other, the voice continuously cuts in and out, which makes the phone call basically worthless. We both have full signal during the call so that isn't an issue. It rarely drops our call completely; it is just an issue with the voice cutting in and out. We can call anyone else with our phones and not have a single problem. Any ideas?
Sounds like the HD voice feature to me. You can test this by contacting customer service (*611 then send from VZW phone or 1-800-922-0204) and asking for them to add the Decline HD voice/advanced calling feature on the lines. Then call each other and see if the issue persists. Of course there is a number of other things that could cause this, but based on your description I believe it has to do with this feature.