Who I have the same questioned this topic

Please help. As of mid Sunday, one particular person (iphone user) can't receive any of my text messages. I can receive all of theirs-- but they can't see mines. We both can receive each other calls and text on the what's up app.. how do i fix this ? We were texting all sunday and all of a sudden they couldn't receive any of my messages. It's just this one particular contact. I have the Note 20. 

thanks