International calls are very expensive in Mexico. If you need to make many international calls you should look into some of the following options to save money.
There is a wide range of alternative prepaid cards for international calls available in Mexico, which often provide significantly cheaper rates than a Telmex phone card. You can buy international calling cards (tarjetas para llamadas a larga distancia) at many shops or online. To use an international calling card you need to dial an access number of the card operator. Operators offer free (01-800) access numbers, normal numbers for which you have to pay a local access fee, or both. Rates can vary depending on which access number you use, so compare the total prices for the access and the international extension.
After dialling the access number you have to dial the PIN-code of your calling card (you should find this code hidden behind a scratch-off panel on the back of the card). Some cards are for single use only, so if you run out of credit you will have to buy a new card (with a new PIN-code). Other cards allow you to top up your credit over the internet which can be handy if you want to keep using the same PIN-code.
Voice over IP calls are a good option for international calls from Mexico if you have high speed internet access. There are many alternative VoIP providers, though the most commonly used is Skype which lets you do both free calls with other Skype users, and low-cost international calls to normal phone numbers. Skype also offers cheap “Skype In” fixed numbers for different countries which are very handy if you want to receive calls from your home country. I.e., if you come from the UK, you can buy a UK fixed line number through Skype, give it to your friends and let them call you in Mexico at the cost of a local call.