If you don't want your caller to have to download an app/service, and you want them to be able to call you with no charges, your best option is to use your current cell phone and just get an international plan. There are options like Talkatone and getting a local number with Skype, etc. but these will only work with a wifi connection unless you get a data plan, and if you pay for an data plan, you will have voice included as well...