Throughout most of S.E. Asia, obtaining a SIM card for making mobile calls is easy and inexpensive. They usually come with prepaid credit as well. This works well is your mobile is unlocked.
The 3 biggest companies in Vietnam are: Mobifone, Viettel, Vinaphone.
1. Mobifone: The cost is: 2,500 VND/text to any country outside Vietnam.(1AUD$ = 21,648) VND) Same for Viettel and Vinaphone.
For inside Vietnam, if you text someone, who also uses Mobifone, it costs like 200 VND/text.
If they use different company like Viettel or Vinaphone, it costs from 250 VND/text.
Mobifone calls Mobifone: 1,600 VND/minute.
Mobifone calls Viettel or else: 1,800 VND/min. Mobifone calls outside Vietnam: 4,200 VND/min
Viettel calls outside VN: 3,600 VND/min
Viettel calls Viettel: 1,200 VND/min
Viettel calls Mobifone or else: 1,400 VND/min
Viettel texts Viettel: 300 VND/txt
Viettel texts Mobi or else: 350 VND/txt
Vina calls outside VN: they have a package, which you can call to the U.S 272 mins and pay just 10$/month and 15$ for 414 mins
SIM cards can be purchased from HCMC airport. There are network stands which are to the right of the Taxi booths by the main exit. Other shops and minimarts also sell them. In Vietnam, SIM cards must be registered to the owner so bring your passport along for identification.
In Thailand, SIM cards can be purchased without identification or registration at any network shops in most shopping malls and at all 7-11s. I usually use 1-2 Call which has good coverage through Thailand and inexpensive overseas rates. To access their cheap rates to calls outside of Thailand, use 00500 before the number.
In Laos, you can use a Thai SIM in most places close to the Thai/Laotian border but for southern Thailand, some people have recommended purchasing a Tigo SIM card for 50,000 kip with 20,000 credit already on. Calls, international or local are just 2,000 kip a min and you get 10 free text messages a day