Phone service losing connection in one direction

Hello - we are having issues with multi-handset phone service losing connection during calls. We cannot hear the person on the other end, but they can still hear us. This happens in the middle of calls. 

A similar issue was reported here - https://forums.xfinity.com/conversations/home-phone-features/my-land-line-keeps-dropping-the-call-in-one-direction-after-a-while/642c59804d04032f94615e6a

the solution in this case was that the person plugged the phone into Port 1 on the modem, but our phone is already connected to Port 1.

Any tips or ideas on how we can fix this? We already purchased a whole new set of handsets and now that we're still having the same issue, it feels like a waste of money.