So I can send SMS messages to everyone and everyone is receiving them without any issues. However a random amount of people (contacts, non-contacts) are telling me they send me an SMS and they get an error not sent. I never receive the messages. I have been through all of the setting, tried different SMS apps, reset the phone and none of it is solving the problem. Worse, since I don't receive only some messages, I do not have the full extent of who is even sending them to me and failing.
But some people have no issues sending them and I cannot made heads or tails of why.
Please advise as this is now affecting my work.