Yea PAC is right.
I know this won't apply to you, as you're going to be switching networks, but for future reference, you can only port your number when switching networks; not if you're staying with your current network provider.
So, for example, if you were with Vodafone and you wanted to start a new contract, you wouldn't be able to port your old number to your new contract. You could obviously upgrade your existing contract, thereby keeping your number, but sometimes that's not possible.
I Know it sounds a bit, but that's how it works .........