Who I have the same questioned this topic

I cannot send or receive texts with one specific number . all texts work to and from everyone else. I am verizon lg g5. The other person is iphone att. I have done factory reset. I have deregistered from imessage both numbers using the apple link. I have tried 3rd party messaging apps. I have deleted the contact and recreated. I have deleted all message threads. Nothing has worked. It is just this one number. I have also adjusted imessage and reset the other persons phone. Deleted and recreated thread and contact on that phone. I am super frustrated. Any other ideas. Thanks