SIM CARD

Officially you can not buy a sim card in Cambodia as you must have an I.D card to get one. However if you could find someone that has an I.D card to buy a sim card for you but expect to pay them a few dollars.

Siem Reap info

In Cambo, ask someone who can speak English to buy you one. It'll cost you about $15. A $5 call card will get you plenty of calls as the average domestic call costs about 10-20 cents.
It's of no use in Malaysia.
I

You CAN legally get a simcard here in Cambodia. A 092 number from Mobitel costs $8. All you need to do is go there with your passport and either a lease on a building (in your name of course) or a letter from your employer (you could write this yourself on a fake letterhead as its not verified).

3# Sorry for not writing it clearly.

I meant as a tourist you can not officially get a sim card in Cambodia. Maybe you are planning to live and work in Cambodia and Malaysia? If you are planning on living in Cambodia or working in Cambodia then you can as 3# states legally get a simcard. I am not sure about Malaysia.

The cost of a SIM card depends on the number, lucky numbers carry a premium. The vender will have a book with all the numbers and the price for each one along side.