Over the last week April 22-27 2019 there was a substantial Cell Phone Update for both my Samsung S8+ and my husbands Moto G6.
We both received a message that our SIM cards were invalid and we were unable to make calls following the update.
I removed the SIM cards from each phone and checked for dust (although there wasn't any) and reinserted the SIM for each phone. That fixed it for some reason.
I just wanted to share in case anyone else had this error. If happened right after the update. If it happened to the two of us it may be a big issue from the update. I hope this helps someone.