I'm currently using www.pincity.com iTalk phone card. They have many local access numbers in Mexico, you can check the local access numbers and rates at http://www.pincity.com/wwlan_rates.asp. They also have a toll free access number in Mexico which can be checked at http://www.pincity.com/itfs_rates.asp.
I suggest you use the local access numbers which provide better rates.
Hope it's useful for you ^-^