The costs of cellular services in Cambodia is extremely cheap. The best way to take advantage of this is to get a local sim card at the airport when you arrive and replace your own card with this one for the duration of your stay in the country.
US$5-10 (for the sim card and airtime) is likely to be all that you will need for ten days in Cambodia, but it can be easily topped up if necessary at a later date. Be sure to discuss whether you will need telephone time or simply data when you purchase this card. If you are only going to be using Whatsapp to make calls to people back home and using social media to document your trip then you will essentially only need data so make sure that it is loaded correctly.
Most of the time the service is 4G and most hotels, restaurants and the like will have free wifi. Just ask for their password.