Not going to make a squat of difference.
Here in Australia, they'll unlock it at the end of your contact, but holy hell, they'll make life unlivable if you try.
My sister tried it some time after the end of her Vodaphone contract with the intent to "hand me down" the phone to mum. Yeah, good luck with that.
They promised to email us the website and details, but after repeating a number of different emails, we were forced to come to the conclusion that either everyone at Voda is dyslexic, or illiterate, or idiots, or all of the above... Don't start me on being policitally incorrect, we were all royally pissed by this stage - was a fucking month and half, three or four email addresses, and stuff knows how many phone calls.
Once we did get to this hitherto mythical website, it didn't work. (not that was hard to use, the numbers just didn't work) More phone calls later, we managed to get them to do it over the phone.
This was good month and a half with (supposedly) dozens of emails back and forth and who knows how many phone calls later (no idea actually, because the calls were split between my sister and myself), it finally worked.
It's easy to say they "allow it", but it DOES NOT mean they'll make it easy for you, of this, I can assure you.
I cannot says this enough NEVER EVER EVER EVER "buy" a phone on contract. Do an outright purchase, or do do without. Don't come crying to me if you can't do anything with your phone outside of your original carrier at the end of your contract. Heard it enough here, and I'm sick of it.