Mexico is country code +52.
The area code for Mexico City is 55, Guadalajara is 33, and Monterrey is 81. All other area codes are three digits in the range 200 through 999. The area code plus local number always totals ten digits.
If you are calling a mobile phone in Mexico, you must insert the prefix '1' in between the country code +52 and the area code. For example, Acapulco is area code 744, so a landline number would begin +52 744, but a mobile phone number would begin +52 1 744.
(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.)
Country code 52 is Mexico.
Leichtenstein is telephone country code +423.
Here is a list of all telephone area codes ("claves LADA") in Mexico, from the Mexican government: http://www.sre.gob.mx/austin/Util/LadasMexico.html (The column labeled "clave actual" is the current area code. The telephone country code for Mexico is +52, and you must also insert the prefix '1' between the country code and the area code when calling a mobile phone or cell phone.)
where is telephone country code 1816
That is not a country code.
Country code +57 is Colombia.
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.