The cheapest method to make international calls is to buy an international phone card (tarjeta internacional). These can be acquired at any ETECSA point of sale office. They cost 10 CUC. These phone cards allow you to use any land line or public phone to make international calls. The instructions on how to use the card are written on the back. You always simply dial the number 166 in order to enter the directory, and then you enter your card number and the pin. Most international calls cost between 1.40 CUC per minute (North America) and 1.50 CUC (rest of the world). While still expensive, this is the cheapest option. All other international calling cards purchased from sources outside the country will not function in Cuba. You can also use your international cards to make local calls, but this will be expensive. The best option for local calls is to use the Tarjetas Propia, as mentioned earlier.
You can also use your cell phone to make calls from Cuba, but this will usually be very expensive. The base rate is about 1.30cuc per minute, plus there will be roaming charges applied from your domestic telecom company. If you have a roaming plan that includes Cuba, then your rates might be slightly lower, but still expensive.