If i buy a UK iPhone 5S unlocked. Can i put a Belgium sim in it?

I live in the UK but my Girlfriend is from Belgium

My contract for my current phone is about to expire and i don't need a new one.

i wanted to get and IPhone 5s on contract and give it to her and keep the sim for my self.

She lives in Belgium and will put a Belgium sim in it. Will it work seeing as i bought it in the UK

the guy at the shop wasn't sure.

It won't work.

"i wanted to get and IPhone 5s on contract" - if you buy it on contract, the iPhone will be locked.

You will have to use one of the third-party services to unlock that phone first. Then you would be able to put any SIM you wish. The UK carrier won't unlock that for you, because you have to have the phone for at least some time and pay off the contract first.

Buying an iPhone unlocked & on contract are 2 different things. (since you stated both in the question).

If it was unlocked then yes it would work but if you are buying on contract it would be locked to the carrier it was bought from & of course would not work with other SIMs. You'd still have to buy the bills for the contract anyway though.

ALL iPhones (except 100% unlocked ones bought at an Apple shop)
lock to the 1st Network /SIM card that is put in them.

Even the ones the network sell as unlocked ones, have in their small print - unlocked to UK networks only (go check)

once locked:
You then need to ask that network to unlock it.
so if it was new, and never saw any other SIM card, she could put her belgium SIM in it
and it would lock to that network - and to get it unlocked, you have to follow that networks unlock rules (and some networks won't unlock!)

To get it unlocked in the UK
get the phone, put a SIM in it
IF the network will unlock it immediately, then OK, (some make you wait 12/24 mnths)
then ask the network to unlock it (pay the 15-20 pounds, - wait 24-48hrs)
and then put a different Network SIM in - that completes the unlock process.

NB Three is the only network currently unlocking iPhones for free
So you could look at putting a THree SIM card in it, as the 1st SIM
and then asking them to unlock it? (but check a PAYG SIM will do that also)

So there you have it.
your call.