Why can't I get texts from one iPhone user?

[AC Question]

Can't get texts from one iPhone user

My dad and I can't send and receive text messages to each other. I have loads of other friends with iPhones and I seem to get their texts just fine. He never gets mine and I never get his. Once we had another iPhone user do a group message we could talk to each other then. Once he was out of the conversation we no longer could. I have never had an iPhone. I do have a Mac Mini and an Apple id.

Any ideas?