It wouldn't either if it fell out of bridge mode, just double-nat'd. Same scenario. Nat loopback doesn't work on Comcast hardware, but if you set it bridge mode and have a router behind it that supports it then it absolutely does work. We do this everyday.

I did that at home and recently for a customer as well.
Owned Modem (Arris bought at BB), Ooma, and choose your own favorite network gear, I use Araknis. Works like a charm, no "rental fees", and pays for it'self (lack of fees) within a year.
*note, always make sure the residential number gets ported before discontinuing service, as others have said. But once done, this works great, and no Xfinity hotspots, no rental fees, no firmware pushes changing settings, and no calls for service upgrades that confuse the customer (until the plant network gets upgraded: DOCIS3.1 for example), until you're ready to sell the client on new gear or services.

Why can't you use 3rd party modem with voice option ? i.e Arris tm822 [Link: amazon.com] or tm822r [Link: amazon.com] + your own router . I had Xfinity X1 service with my own router and modem with telephone line ( arris tm822g ) at home over the last two years, never had any issues. Yes, there was some complaining from the cable guy at the time it was installed but that's about it.

At bare minimum it would require a splitter and the use of separate modems (one owned for internet and one for phone). Locally, Comcast won't connect or activate owner supplied phone modems, so you're still stuck leasing from them. In the end it doesn't matter what "device" you use; Oome, Skype, Ubiquity phone, etc... even the Comcast box, they are all IP phone services anyway. Comcast just wants the billing..

They really can't do it, it's againts FCC law. If they give you hard time, ask for a supervisor. Don't take no for an answer. They even have unique Comcast "approved" modem with voice [Link: surfboard.com] as well as dedicated info page on their website [Link: xfinity.com] .
99% of the time is just a case of a lazy/un-informed installer or tech support agent who does not feel like helping their client with some non-standard installation.