My daughter and I are on the same plan. When I text her, I get the message back, and it never gets to her. I have tried turning my phone off and then on, and re-entering her phone number under the contacts. When she texts me, I get her message.
I ended up trying a reset all settings under General> Reset> Reset all Settings. This seemed to work. Not sure what I did to get it the way it was going before. I have had this phone for a year and never did that before.