How to switch from Fios phone service to Xfinity cable/modem phone service

We recently switched from Ziply Fiber (previously GTE/Verizon/Frontier) Fios phone service to Xfinity, along with internet and TV. The service tech that came for our installation in hindsight was not familiar with how to switch the phone service over, and simply connected the new modem to our nearest wall phone jack to theoretically back feed all the other phone jacks in the house. The phones still worked because the Ziply service had not been cancelled yet. Once the phone number was ported over to Xfinity and the Ziply service was cancelled, the phones no longer worked. I was able to figure out myself to do 2 things that allowed everything to work properly: 1) Unplug the Fios power supply & backup battery that powered the Ziply phone lines. (In our case this was on an inside garage wall.) 2.) Open the ONT (Optical Network Terminal) box on the outside of the house, and disconnect the two telephone wires from the red & green terminals visible as soon as you open the cover. Those two steps were all it took to get our new Xfinity phone service working perfectly.  Questions welcome.