I can't send or receive text messages from 2 contacts

I have a weird situation that just started two days ago. I am not able to send or receive text messages from 2 of my contacts. One is my husband and the other is a close friend -- I have texted them often and recently (until 2 days ago). I get a Not Delivered message, and when I tap the "!" to Try Again, it doesn't help. I have tried all the troubleshooting I can find to do, including Chat with Apple Support. I have restarted my phone (several times), I have removed the text threads for both of these contacts. I have removed them as contacts and re-added them. I have Reset Network Settings. I have checked the Settings>Messages and toggled all of them off/on. I currently have iMessage, SMS, MMS and Group Messaging in the 'on' position. I checked my Blocked Contacts and they are not in there (I am also not in their blocked contacts, and they have both removed me as a contact and deleted all of our past text conversations). I did the update for iOS 15.5. Apple Support ran a diagnostics check and said my phone seems fine. Even more strange, today, I was able to text both of these people if I did a group text -- I sent a text to my husband and myself...he received it. But, I did not receive his reply. Again, this just started 2 days ago. I'm completely stumped....