Have you tried contacting BT and asking them if it is possible to use their SIM card in an Orange phone ' they will tell you if it is possible (they are more likely to than Orange anyway). Otherwise there is a user guide to the Nokia 3210 online at http://www.orange.co.uk/owner/howto/products/phone s/nokia/nk3210e/
- it may tell you how to unlock it here. Otherwise I can only tell you how to unlock the key pad (press the menu button, then # to lock the keypad. Then repeat this process to unlock it) not the SIM card.
Orange and Cellnet r not compatible. U need to have the unlocking codes and leads to do it through a pc or take it to a small mobile shop or market stall type thing, you can get most nokias unlocked for about��10/15. Or try the auctions on Yahoo/EBay, some sellers do a mail order serv. Sending phone to Orange will cost about �40!