How can I find the origin of an international number which appears on the caller id?

Someone called me from a number I could not identify. The number is 12 digits therefore it cannot be a local number. When I check the first 3 digits it looks like Czech Republic but I know nobody there. When I call the number back, I get the operator which says (in English, with a British accent) 'please enter the local number preceded with 0 or your pin to dial an outside number' or something similar. What can this be? Does anybody know?