Where is area code 990?

There is no area code +1 990 in North America (USA, Canada, etc.). North American area codes cannot have '9' as the middle digit.

There is also no area code 990 in Mexico.

There is no international telephone country code +990, nor any valid international telephone numbers beginning with +990.

(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.)