The international dialing code for Cambodia is +855. When making international phone calls from Cambodia, dial 007 or 001 + country code + area code + telephone number.
Many Internet cafes in Siem Reap and Phnom Penh provide a fast and inexpensive net phone service. Here is the list of area codes of major tourist cities;

Phnom Penh - (0) 23                        Sihanoukville – (0) 34
Siem Reap - (0) 63                           Ko Kong – (0) 35
Battambang – (0) 53   

At the time of writing, domestic calls in the same area cost 500 Riel per minute and local long-distance calls is 1 USD per minute. Telephone numbers in Cambodia sometimes begin with ‘0’. It is necessary to dial ‘0’ at the beginning before the rest of the number (e.g. 023-000000).