Pavel, Colleagues,
My mobile carrier is MTS (Russia) provide me Nano-SIM and iPhone 5 works fine with GSM Voice, EDGE and 3G mobile data. Now I am in roaming and no issue with services I mentioned.
BUT as I wrote before Then MTS support line provide me procedure of fixing message center number. Same way as you did. Even I can see correct messaging center number (+79168999100 for MTS Russia) after that I still can't send SMS.
Dear Imobl,
Sure when you check in Settings=>General=>Carrier you see local GSM carrier name. Actually in Russia it was "MTS", in roaming now it is "KZ KCELL" (GSM operator in Kazakhstan).
Regarding problem: To me it seams that programmer writing iOS 6 code in "Sprint" version does not check the value in settings. He just have the Sprint messaging center number in code.
For my understanding It is not a "lock behavior" in any form. Locked phone shouldnot allow GSM radio come up and work for main services. It just a bug for my understanding.
/Ilgar