from the 3 website
'There are usually two restrictions on using your phone abroad - one phone specific and one network specific. If your phone is Quad band that means you can use it anywhere in the world (almost!), if your phone is Tri-band then you can use it within Europe. We list whether each phone is quad-band or tri-band in its individual specifications list.
As the allowances with your contract are usually only applicable within the UK, Three will usually charge roaming rates when you call to or from abroad. Details of roaming charges, including data charges, can be found in our Price Guide. You can check to see if your phone is enabled to roam by logging into your My3 account, which you can register for once you have a Three device. If you would like to turn off international roaming, you can also do this through your My3 account.'
Like any other network, if you didn;t set up the international roaming and buy a package before you went you are paying over the odds and may well have run up a huge bill