SOS means that your phone is getting a signal but TMO doesn't have a roaming agreement with that carrier or region or whatever. Who knows what kind of signal you are getting though. Could be GSM, grps, GPRS,edge or EDGE which means you could have data or just be able to make phone calls.
But what you can do is go into your options, then mobile network where you can manually pick your carriers. After you search, you will be able to tell what mobile carrier you are picking up. This way when you talk to TMO CS you can find out if they have a roaming agreement with that provider.