Cannot send text messages to a certain person

I cannot messages to only one person on my phone. It started yesterday and I haven't had this problem before. They can only receive photos but not text messages. When I try calling them, it goes straight to voicemail. But I can receive there call just fine and pick up the phone. I've talked to 3 AT&T representatives, and all their all suggestions didn't work for me. I did a force restart, turned it it off normally, and reset network settings. I even did a factory reset on my iPhone. The person I'm trying to talk to said they didn't accidentally block me and I could message just fine the day before