When calling internationally, you must first enter an international access code to tell the phone system you want to place an international call. The good news is that + is the global international access code for cell phones. However, if using a landline, you need to know the specific access code to get out of your country: 011 calling from US & Canada; 00 calling from Mexico.
After entering the international access code, then enter the country code: 1 calling to US & Canada; 52 calling to Mexico.

Exceptions:
 1. a US or Canadian cell phone in Mexico; call this phone as if it is in its country of origin.
 2. Mexican 800 numbers. I have found no way to call a Mexican 800 number from another country.