Telmex/Ladatel payphone booths all over Mexico now accept Ladatel prepaid calling cards. You can purchase them at most stores (corner markets, newsstands, pharmacies, etc.) in amounts ranging from 30 to 100 pesos. A store definitely sells the cards if it has a big "TELMEX" sign in front. Calls are very inexpensive, and usually range between 1 and 4 pesos per minute if you are calling within Mexico. Just remember that if you are calling TO a Mexican cell phone you now have to pay a government surcharge that ranges from 14 to 18 pesos per call, which will be deducted from your calling card.
The Telcel pre-paid calling plan for cell phones in Mexico is called Amigo. Just like with pay phones, you can purchase prepaid cards in various amounts for your Mexican cell phone (or your US cell phone reformatted for use in Mexico). The main catch here is that you have to use all of your minutes by the expiration date on the card, which means that even if you have a lot of time left on the card, if it has expired, you are out of luck and have to buy a new one.
There are a lot of benefits to using pre-paid calling cards. Especially for people who like to talk a lot and lose track of time, using a calling card helps you not to spend more money than you originally planned. Phone cards are also extremely convenient because you can use them from any land line or cell phone, wherever you are, without needing permission from your local or long distance carrier, and you don’t have to worry about "surprise" long distance roaming charges.