Where is telephone country code 359 88?

Telephone country code +359 is Bulgaria. A number beginning +359 88 is a GSM mobile phone on the MTel network. The rest of the number is 7 further digits.

(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.)