If I were in this situation (which I assume to be that you want to be able for people to be easily able to get to you in both the US and Mexico), here's what I'd do:

Find a VoIP provider that has numbers in Mexico and in the US. Get an account with them and two numbers, one in the US, one in Mexico. Get a US prepaid cellphone and a Mexico prepaid cellphone. Set up your VoIP service to forward to whichever phone is in the country you're going to be in. When you head for the other country, change the forward to the other phone. Don't give out either cellphone number; use the VoIP numbers exclusively.

This results in people in Mexico being able to use a Mexican number, people in the US being able to use a US number, and you being able to get your calls no matter which country you're in, without any exorbitant fees.

Although, I don't know for sure that any VoIP provider is going to have both US and Mexican numbers. But that would certainly be my starting point.