Old number to new micro sim?

I just got a new iphone and for me it's a huge update since my old phone is a nokia3310. Last week I activate a new micro sim card (t-mobile), but I'd like to keep my old number (orange).
I don't know how to do it! I've look on the internet, but seriously is really confusing to me! I've tried to call the 150 both from my old and the new number, but I don't know what do…
Could someone help me please?
Thanks

Added (1). I don't wanna cut it! Plus i made a contract on the new number

Cut your old Sim Card with Micro-sim cutter or do it manually. Here's the link:

http://howto.cnet.com/...-sim-card/

Call you Carrier, Someone should be able to assect you in switching over. That how I keep the same number any time I buy a new Phone.