Can I keep my number when buying an iPhone?

Hi, I currently have a htc desire c and thinking of getting an iPhone (either 4s, 5c or 5s - if it makes a difference) …
Anyway I was wondering could I keep my old phone number from my htc (or keep my old SIM card) when I get the iPhone or would I have to have a completely different SIM card and number?

You need to tell your carrier you wish to keep the number you already have. If it is a new carrier you will need to ask for a PAK code, they do not charge you anything for it either.

Just tell your new carrier that you want to port your old number to your new service. It should not present a problem.

You can keep the number for a fee as long as you don't switch carriers.

I don't see why not. Just a matter of swapping the SIM from one into the other, assuming you are sticking with the same carrier (or are on a GSM one).

The other thing could be the SIM size since the 4s uses microSIMs & the 5 on up use nanoSIMs which you could either cut to size or get a SIM with your number on it in that size.