BM08

Because it was on tmobile when i bought it and ive just got an o2 sim card to try(pay as you go), put it in and it all comes on, signal bar was full but it said limited service, does that mean its just because i need to top up or is it because my phone needs to be unlocked?

I think its just i need to top up because the signal bar wouldnt be full if it didnt work would it? and also it has a few o2 numbers in my contact list.