I had this set-up and problem. Use one xfinity voice line. Out of the back of the modem, I had one phone line splitter. One side of the splitter went directly to an office phone, the other side of the splitter went into a wall phone jack to energize other jacks around the house. Everything worked fine, until one day it didn't.
rebooted router with splitter in place
changed splitter with two phone cables
plugged phone cable that energized phone jacks directly into modem, would get a breif dial tone, then fast busy, then nothing
plugged phone cable from office phone directly into modem worked
Turns out that I had a short in my home wiring. Some of the phone wires in the interface box weren't properly terminated, and must have moved enough (from wind, leaf blower nearby, who knows?) that they were creating a short.
Comcast tech terminated the wires in the box, now everything works properly. Although still have an ever present hum in the line on both phones.
So, nothing had changed that I could figure out. One day it worked, and then another day it didn't.
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