I'm traveling to Yogyakarta for a 1-week stay, transiting through SIN and CGK. I am looking for recommendations regarding a good cell provider to use, and where I can purchase a SIM for my phone (no data needed). As I will be in SIN and CGK for several hours, if there is a good place airside to purchase, that would be a plus. If not, where in Yogyakarta? Thanks for your insights!