Where is country code plus 674?

Country code +674 is the tiny Pacific island nation of Nauru. A number beginning with +674 559 is a mobile phone. There are 4 further digits, for a total of 7 digits in the subscriber number in Nauru.

(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, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)