I used to have the same problem right after I bought the phone. Back then I had it on 1.6.
The issue is that for some unknown reason (unknown to my warranty provider also) the phone can't read the SMSC from the SIM card. But before they realised that they tested a SIM card which worked ok, even if I tested many and none worked. They said I should change my SIM to an older platform (SIM platform) and it should work. Then they played around with some other SIM cards and got it to work but they don't know why or how they did it.
Since then I updated and reflashed the phone a couple of times and it's still ok.
Better talk to SE Support about this, since your warranty should be void because of the unlocking. But i'm not shure.