The Samsung Focus comes with the "SMS Center Number" of: +13123149810
I resolved the issue by using: +19703769301
It would be better if AT&T gave better definition around which users should use which numbers depending upon how long they have been with AT&T. Fortunately, there's not that many numbers to try.
In case AT&T ever deletes the link above, here's what it says:
Advisory: IMPORTANT: A plus "+" sign MUST precede the 11-digit message center number in order for Text Messaging to function. Instructions: Existing and New AT&T customers The message center number for all existing and new AT&T SIM cards is +13123149810. If experiencing problems with text messaging, please contact Customer Care.
Former AT&T Wireless customers The message center number for all former AT&T Wireless customers is +19703769301.
Former Cellular One from Dobsoncustomers
The message center number for all former Cellular One from Dobson customers is +13305310005.
Content reflects instructions for devices and services from AT&T. Some differences may exist for devices not purchased from AT&T.