Are they actually Apple users though. I had this issue when a family member switched to Android for a while (now back on iPhone thankfully). It saw that they had an Apple account and no matter what I did kept trying to send messages using iMessage instead of SMS so none of my messages made it to them but theirs came through to me. They had to do something on their old iPhone to fix it.
LisaQuinnYT t1_j9oqggm wrote
Reply to My phones not letting me send any messages to Apple users but working just fine with android users I’ve tried resetting it and going to messages settings!?(sorry for my bad spelling I was worried) by Worldly-Salamander35
Are they actually Apple users though. I had this issue when a family member switched to Android for a while (now back on iPhone thankfully). It saw that they had an Apple account and no matter what I did kept trying to send messages using iMessage instead of SMS so none of my messages made it to them but theirs came through to me. They had to do something on their old iPhone to fix it.