Telephone country codes never begin with zero.
Country code +421, dialed as 00 421 from many places, is Slovakia.
Country code +21, dialed as 004 21 from many places, is an incomplete code in Africa. For example, +211 is South Sudan and +212 is Morocco.
To avoid confusion, it is best to write an international telephone number in correct international format, beginning with the plus symbol and the correct telephone country code (e.g., +421 or +211), omitting any dialing prefix.
(The plus signmeans "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.)
The calling code 0042 is not currently assigned to any country. It was previously used by Czechoslovakia before its dissolution in 1993. After that, the country codes for the Czech Republic and Slovakia were assigned as +420 and +421, respectively. If you're seeing the code 0042, it may be an outdated reference or a misinterpretation.
Leichtenstein is telephone country code +423.
where is telephone country code 1816
That is not a country code.
Country code +57 is Colombia.
Country code 52 is Mexico.
Fiji has telephone country code +679.
Gibraltar is country code +350.
Moldova is country code +373.
Slovenia is country code +386.
Vanuatu is country code +678.
Niue is country code +683.