Where is telephone country code 42?

Country code +42 (dialed as 00 42 from many places) was Czechoslovakia. When that country split, the Czech Republic became country code +420 and Slovakia became country code +421. Country code +423 was assigned to Liechtenstein. The remaining +42X codes are not yet in use.

(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.)