Where is phone area code 300?

The number 300 is not a valid area code in North America (USA, Canada, etc.; country code +1) or in Mexico (+52) or in the UK (+44) or in Australia (+61) or in Russia (+7). There may be an area code 300 somewhere, though.

Country code +300 also does not exist. Country code +30 is Greece, but no valid numbers begin with +30 0.

(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.)