How to fix an IPhone 4 not sending text messages?

I entered my Apple ID on my dads phone the other day in attempt to download the Facebook app for him. Our internet connection was to slow so it didn't work. I signed out of my Apple ID on his phone and now he can't send text messages, he can receive them, but not send.
Any suggestions? I tried restarting it a few times, but that didn't work.

You can reset all setting from setting option. Setting > General > Reset > Reset All Setting. Then your problem will be solved.