I recently upgraded my internal SSD on my MacPro 2013 and put the old SSD into a special USB enclosure that I can boot off of. When I boot from the old SSD, it boots into Mohave (so I can use certain software that doesn't work with Catalina). When I boot back into Catalina, some of my text contacts will no longer send or receive, but others do. Usually, after a couple of weeks, they start working again on their own.
Someone suggested I boot with the Shift key down to see if it might be an extension causing the problem. I tried that a while back and iMessage did work correctly that way. But I never figured out which extension was the culprit.
Just thought I would post this in case anyone might think of something to narrow it down even more.