Can Iphone 3G be (unlocked) to to another network (rogers --> fido) ?

So basically I bought an iphone 3G (8GB) which was used... the mistake i made was to check what network it was under previously... so i bought it and now my sim card wont work in it.

I know someone who says they can unlock the phone from rogers to fido (which are canadian cell phone companies) just checking if this can be done?