Some mobile phones are very difficult to unlock, especially complicated ones. I was given a Nokia N95 on T-Mobile and bought an X-SIM off ebay. This is placed between the SIM and the phone's contacts and effectively unlocks the phone for use by any SIM. I paid under £4 delivered.
Mine also works on HTC TyTN II and T-Mobile MDA Touch (which is VERY difficult to unlock). The X-SIM comes pre-programmed but also supplied an extensive list of additional code for other phones. All my phones unlocked with the default setting click here
thaks for all the replies, its a bit of a gadget- feels like more of a mini pc tablet than standard phone with its tiny pop out keyboard, only useful for some quick net if u have tiny fingers, lol. this phone was given by a relative, so more of a present than anything. im not sure if the phone itself needs to be unlocked for an "unlocked sim card for n900" to work- there are a few of these advertised on ebay.
they are around a tenner, so I could try one of those and cross my fingers that it works. i cant see anything on the phone that tells me it will only work with a certain company for sim cards- although even without a sim card i have managed to set it up for some free wifi net with a family BT router in a test. no doubt that probably tells me little about sim compatibility as im assuming as long as i have the wifi key any net service would work with it.