Can I just add a word of caution..? Many of the SIM cards mentioned above give you access to the internet, i.e. they are for "data download". You can't actually phone anyone with them. There are a couple of advantages, however, to data download : first, you need to use WhatsApp (or similar) but keep your regular HK telephone number, so you aren't just in contact with US, but with the whole world. If you use voice on WhatsApp, then you can "phone" people. The 2nd advantage is that this sort of card works anywhere in the world (within reason), not just the USA. SmarTone sells 30-day cards, 1.5GBs, for HK$298, although there will probably be cheaper versions elsewhere.