HTC Incredible S - Can't seem to get another sim to work!

[ChrisMayhew]

Hello

I've been trying for a while to get a Tesco Mobile sim card to work in my HTC Incredible S.. It isn't telling me it's Sim Locked and isn't asking for a code to unlock?
It will just not get a signal at all.. Using a Tesco mobile or Orange sim.

The phone was originally bought through Carphone Warehouse on Orange.

I'm wondering if anyone has any ideas why it wouldn't be able to get a signal? Maybe it's just broke? lol

Thanks,
Chris