Currently Being ModeratedMar 9, 2014 9:48 AM (in response to scorpiono1)
I just experienced the same problem with my 4s. What compounded my issue was I couldn't turn off my phone for a system reset due to the on/off button at the top of the phone being broken (3 days after the warranty expired, go figure). Anyway, I called my provider to see if anything was wrong with the account. They assured me nothing was wrong. I explained that I was in the USA for about 24 hours roaming off AT&T's network. When I crossed back into Canada, the phone wouldn't detect my carrier or any other. Long story short, after a little investigating, just do a rest network settings from the general menu. Waited for the phone to reset and voila! Back to my carrier.
Just sharing the experience in hopes it solves other people's problems.