"My wife was talking about a phone to have around, but she doesn't like Xfinity VoIP phone service, so can look around online for VoIP services."
Before I worked for Comcast, as a network engineer, I've used 3rd party VoIP services across Comcast cable, and also worked with another network engineer (also non Comcast) who did too.
In my case, as only I used my home Internet, VoIP, by itself, worked just fine.
My co-worker, though, had a family setup, sharing the Comcast cable link and found he had to have QoS prioritize 3rd party VoIP traffic to Comcast, otherwise any other activity, using the Internet in the household, tended to make VoIP unusable.
Comcast cable is (comparatively) very limited in the bandwidth it provides "up". Traditionally that bandwidth is also "shared" with your neighbors. Their dedicated VoIP service has its own allocated bandwidth in their infrastructure. I.e. even with QoS prioritization on your 3rd party VoIP traffic to the Internet, you can bump into VoIP issues due to bandwidth shared (with your neighbors) "up" bandwidth.
As the time I left Comcast, they were extending bandwidth closer to end users, using optical fiber, but I'm not current on how much has changed. (I believe they still like to "advertise" their "down" bandwidth, but they are a bit mum about what a Internet plan provides for "up" bandwidth. Both, though, just with many other home (often business too) Internet providers, bandwidth is often an "up to", i.e. no actual service guarantee. Again, for Comcast's VoIP offering, they insure it always has the bandwidth it needs; not the case for 3rd party VoIP.
