One more thought. Think long term. If you're likely to travel again and might need a phone again, buying one will be even more cost effective. And buying one after you arrive will be least expensive. Phone service in the US is notoriously expensive.
People in developing countries all have mobile phones because of the lack of infrastructure for land lines. So low-income working people all over the world rely on cell service and it must be cheap. So doing as the locals do when you travel beyond western countries is just the sensible thing to do. With phone service as in other things.