I have setup my Windows 10 phone using U.S. as the default location, so that I could use Cortana (currently not supported in Greece). I also enabled international assist for calling, as I travel quite often outside Greece.
Today I tried to call a landline phone on Greece. I received an automated response about the subscriber not being available – in English. I checked the number I have called only to see that it has been corrected to U.S. number and in fact I had called a U.S. number.
This costed me about a couple of euros for a 10 second call. A lesson learned – I disabled the international assist option.