Replacing the Xfinity modem with your own modem

I got tired of my Xfinity bill getting so expensive and called Xfinity Customer support to talk about it, and they suggested I can significantly reduce cost by eliminating all the equipment I was renting from Comcast. All I had to do was return the equipment and purchase my own modem and just use the "Xfinity Stream" App.  I followed their suggestion and purchased the Motorola MT8733 that was recommended as one of the devices for the Xfinity Internet Plan.  I picked this device because it supports WiFi, a "Voice Telephone", and DOCSIS 3.1 - which seemed to be functionally equivalent to the XB7 I was renting.  After a couple of fixes, I can say the "Xfinity Stream" functionality meets my expectation.      

However, I was having trouble getting my "Voice Telephone" (aka Land Line) working with the new modem.   Specifically, I could receive incoming calls with no issue, but I could not make an outgoing call as it just results in a busy signal. I spent a couple of weeks working with both Motorola support and Xfinity support trying to resolve this issue. The most frustrating part of that exercise is the finger pointing, where Motorola says the problem is with Xfinity and Xfinity says the problem is with Motorola. I have to say Xfinity seems to be winning the argument because they swapped the MT8733 with their XB8 and demonstrated my Land Line phone works in both directions - but when I shared that information with Motorola they were not surprised (I am not sure what they mean by that).

Motorola tells me there are other customers that have accomplished what I am trying to do, and have suggested reaching out in this forum for advice. Is there anyone on this forum that has had success in replacing the Xfinity modem with their own modem to support Xfinity Stream and a Voice Telephone?  If so, please share what modem you are using and please share any special steps needed to get your Land Line working as expected.