Where is telephone country code 00?

There is no telephone country code +00. Country codes never begin with zero.

00 is the most common access prefix for international calls. If 00 is your international access prefix, then dial 00 followed by the country code, city code (if applicable), and subscriber number.

(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.)