Yes, you should be able to use your mobile/cellphone in Cambodia, as coverage is generally quite widespread in the country's larger cities and towns, but may not be available in rural areas. Global roaming can be very expensive in Cambodia, so the best option is to buy a local SIM if you want to use your phone on the go.
Buying a SIM card in Cambodia
To avoid roaming charges, it is possible to buy a local SIM card in Cambodia. Local SIM cards can be used on most unlocked phones from Europe, Asia and Australia and some unlocked phones from North America.
Cambodia has a number of service providers, including Cellcard/Mobitel, Metfone, Smart and CooTel. Each company varies in terms of offerings and price, but all have generally good coverage in the main cities like Phnom Penh and Siem Reap. There are a number of ways to purchase a SIM card, but the best (and cheapest) way to do so is go straight to the mobile company and buy from there. Otherwise, pick one up at the arrivals hall in the airport. A service representative may need to take a scanned photo of your passport to verify the purchase in both instances. Cellcard offers prepaid SIMs either as pay-as-you-go or in packages, with calls and 3 GB of mobile data for around USD 5.
Keep in mind that if you have come from Vietnam and have a compatible Viettel SIM card, this will probably work in Cambodia too!
Our tours in Cambodia