International Calling outside of North America is disabled by default on Snapcom but can be enabled by submitting a request.
If you have international calling enabled, you can use the following format to dial internationally:
011(international call prefix) + country code + number
For example, to call the US Embassy in the UK I would dial:
011 44 20 7499 9000
011 is the international prefix
44 is the country code
2074999000 is the number