There are phone booths available in the big cities where you can make domestic calls. You can also buy sim-cards to use your cell phones here or you can simply activate your international roaming.
Some calling services do offer an option of making free calls between various countries.
However, there are many internet cafes and coffee shops which will allow you to use the internet. You can make international calls from these internet cafes for free or at a very cheap rate.
You can also make calls from your hote. Post offices can be also found easily and you can make your domestic/international posts at any of them using a variety of systems.