I think any of these are very expensive ways to do calling for MOST countries. I was able to by a SIM in the arrivals lobby of Hanoi airport and use it in my phone with no problem. I suspect they are easy to get in Bangkok as well. I dont think prepaids are sold in Cambodia, at least neither myself nor our guide in Siem Reap could find one. Cambodia is an exception if that is true though. In most countries you are much better off just buying one when you get there.