AT&T error message 1111 when sending SMS messages

[LandLockedPH]

Got my 5T today and love it, however; I'm now getting the above error message when trying to text. It worked at first but now no longer sends outgoing text messages. I am able to receive. Anybody?

 

[LLcdPH]

Discovered that the correct message center number was missing. Found and entered it and now texting correctly.