I've just got an new phone and it came with a 3G sim card, but I wanna keep my old number, however my old sim card isn't 3G, can I still use the old sim in the new phone?
The new 3G sim that you received wont have a number attached to it until you activate it. Just call your customer service with the serial number and they will activate it and your number will be on the new sim card.
If you dont then you wont be able to use any of the 3G features of your handset.