sounds similar to the very popular WhatsApp, (which we use quite a bit)...but are you sure it's free to call any number in SXM from the US, or just calls using the app, one Talkatone user to anotherTalkatone user?...although very economical, i do see international rates to SXM: Talkatone Rates - US to Netherland Antilles

as far as calls TO the US, most US mobile plans + supported phones can use WiFi calling to make & receive free calls to/from any number in the US