If you are an AT&T Wireless mobile network customer and would like to update or change the AT&T message center number based on your region or state see below.
If SMS is not working one of the problems could be the wrong SMS Message Center Number.
AT&T Message Center Number
The default Message Center Number for all AT&T SIM cards is +13123149810 or +1.312.314.9810
The above message center number is the same nationally.
To know the message center number of your region follow the below process
Go to att.com and look for the Chat option on the right side
You can Call 611 from an AT&T wireless phone.
You can Call 800.331.0500 (For AT&T PREPAID customers call 800.901.9878)