What country has an area code 307?

In North America, area code +1 307 is the U.S. state of Wyoming.

Country code +30 is Greece, but there are no valid numbers beginning +30 7.

Other countries may have domestic area codes like 307.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, and many other countries use different prefixes.)