Computer Help and Support

I had comcast in the past.
I had a modem from when I bought my house 11 years ago until 3 years ago. In the 18 months after that I had 3 different "newer" "better" modems which crapped out within months install.
I gave up on comcast.

IMO you have an intermittent problem that has to do some handshaking between the modem and the cordless base unit. If you have no alternative to comcast, you could install a small UPS that would keep both units alive in the event of momentary power drops.

I had something similar to this:
https://www.newegg.com/cyberpower-cp350slg-nema-5-15r/p/N82E16842102135

Personally, I would tell comcast to unbundle the VOIP demand they cut your bill by the amount they charge for VOIP and switch providers:
https://www.zdnet.com/article/best-voip-providers/

About 90% of support techs in many industries have little technical training and mostly rely on a script to guide them.
Trying to get one of these guys to realize that an intermittent problem may actually be a problem that they can't verify is a losing battle.