I truly wonder who was the idiot who decided on the dialing scheme in use in Mexico. Absolutely user-unfriendly, it means that a person roaming there doesn't actually know how to dial a number as the standard international form (country code + area/mobile code + number such as +52 55 5555-5555) just doesn't work. To make matters worse, the number you dial to a mobile phone changes every time depending on where the caller and the receiver are. Absolutely insane!
The details in Lee Iwan's blog.