I got upgraded to iPhone 5 with a new sim but the number is not the same?

They said to me that I would be using the same number but when it came today there's a different number. I don't get it because I'm on a contract and my old sim is still working fine and I have this yet I only want one number and one contract. Is it a mistake or can they T-Mobile help me?

Call up T-Mobile and ask them to transfer your old number onto your new sim. They should help you with it easily