Can I use a t-mobile contract free iphone 6 in europe with a sim card from a local provider? Or: How do I know if my country has a compatible network? A friend of mine is going to NYC. I'd like him to buy a contract free t-mobile iphone 6 for me cause they are cheaper in the US. I would like to use this phone with my european sim card provider. Is that possible?

When I've googled this I've seen some contradictory information on forums (can't post more than two links here cause I'm new) and in various websites. An apple customer rep told me: "As long as your preferred wireless provider is compatible with the T-Mobile GSM network, then it will work." What does compatible mean here? Is it a matter of frequencies and stuff that is fairly standardised? If I'd be able to connect to the local carriers network (ie. roam) with a t-mobile sim while travelling does that mean the network is compatible?

It seems like you might be able to discover if you're phone is useable in other countries for sure by looking at the per country coverage lists you'll find if you google "International Plans Calling Abroad Traveling T-Mobile".

The question "When I buy an iPhone 6 from the Apple website and select contract-free from T-Mobile, is the phone carrier unlocked?" that's already available on this page might be seen as an answer to this question. However clarifying what "compatible carriers" mean and wether information on where roaming is possible answers the question "are there compatible carriers in my country?" might be nice for other users, at least it'd be nice for me.

http://store.apple.com/us/question/answers/iphone/is-iphone-6-for-tmobile-unlocked/QU2A7TAHD7HYCJPD9

http://hothardware.com/News/Want-An-Unlocked-iPhone-6-Pick-Up-The-TMobile-Version-ContractFree/

EDIT:

For LTE coverage info, I think you can compare supported LTE bands on the different models of iphones with LTE bands on networks in your country: 

http://en.wikipedia.org/wiki/IPhone_6 (see LTE bands)
http://en.wikipedia.org/wiki/List_of_LTE_networks (look for your country)
The phone will work (all the typical phone functions) without any problems. From what I've heard, it is only unlocked if you buy the T-Mobile version directly from the Apple store (not from a T-Mobile store). 

As mentioned above, the LTE functionality is the question. Apple's website details the T-Mobile version's included LTE bands (model A1549 (GSM)).

You can check those bands versus what bands your carrier uses to determine if it will give you the full LTE functionality. Wikipedia has an extensive LTE band list per country.