Phone_Zero,
That number should be correct.
Here's another area that you should check.
Main Menu->Settings->Connectivity->Data Comm->SMS Access
Make sure that it is set to circuit switched.
If that doesn't work, try a Master Reset->Reset Settings.
If that doesn't do it and you say that the service center number in your text message settings is +13123149810, then I am out of ideas, but try calling customer support for help and to verify that the service center number is right because it may be different depending on your area too.
By the way, when you got your phone was it CID53? Or did you change it to CID53 during the debranding process?