The fact still remains that taking your mobile phone overseas and “roaming” is very expensive. Nice to see that Vodafone and Spark have a $5/day deal where you end up using your own minutes and data but the cheapest option is still to buy a local sim card.
The first step is to make sure your phone is not locked. Call your provider and double check. Next, unlock it.
This site here gives a good background of how it all works.
The key nowadays is data. Once you have data you can Uber, Viber, Whatsapp, Skype etc etc. With free wifi very common in major cities it means you can conserve your data also.