IPhone messages won't send to ONE person?

My dad cannot message me at all. We both have iPhone 5s', however he still uses SMS, while I use iMessage. This shouldn't be a problem since iPhone's automatically switches the SMS/iMessage mode depending on the person who you're talking with. Every time my dad tries to message me, it will not go through. He gets a red statement under his message saying "Not Delivered" with a red exclamation mark.

I checked both iPhone's blocked menu, and we didn't block each other. I can message him fine. So I did some testing like deleting my contact from his phone completely. Then I tried messaging him, and I expected to see 1(###) ###-#### as the contact name on his iPhone, however I got instead 001##########. I'm curious to know if those 00 in the beginning means something. If so, how do I remove it? I did some research and it means some out of the country type of number.

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