iphone 3gs problem with sim card

[honey33]

i have a htc wildfire with 02 paygo sim card in it and works fine but now i have a i phone 3gs and when i put sim card in it say it is not compatible, yet the person i got the iphone of had a 02 sim in it but not sure if it was pay as u go...basically i top up 15 pound a month and gives me free internet text etc...so why is it saying it not compatable any help would be great, i have tried my wifes orange pay go sim in it and that seems to work fine......is there some sort of sync issue cos it was in htc first or something daft like that