Can I use the Iphone 6 in Belgium?

You might need to wait until the unlocked version of the iPhone is released by Apple, probably in about 2 months time.

If you buy the contract free T-Mobile phone in the US, then you'll need to activate it on T-Mobile and then get it unlocked before you go to Belgium, so that you can then insert a Belgium sim card into the phone.

However, be aware that if you buy it in the US, you won't have any warranty cover for the phone in Belgium - iPhone warranty is not international, so you'll need to bring the phone back to the US in person. Also, it won't work with 4G/LTE in Belgium either.

If you are only a student in the US, then buy a cheap phone to use whilst you are there and then buy a new iPhone when you return to Belgium. If you do this, then you'll have EU warranty on the phone and it will work on 4G too.