I should point out that you can buy time two ways: 1) the card, which can be bought a many of the small stores and at Telcel distributors. You key in the number on the back of the card after dialing a special number. 2) direct buy at ATMs, supermarkets, etc. Often supermarkets have special discounts. For example,
Chedraui sells 500 pesos worth, but gives you 900 pesos worth with similar discount ...