Communications infrastructure was devastated during the long civil war but the cell phone has allowed Cambodia to catch up. Almost everyone has a cell phone, even the tuk-tuk drivers, and Cambodian cell numbers start with 01 or 09. You can make calls on international roaming but be prepared for punitive Cambodian roaming rates.
Buy a SIM Card
A good strategy is to buy a Cambodian SIM card for use in your unlocked cell phone or have the local shops unlock your phone for a small fee. To buy a SIM card, you may have to show some proof of residency or find a Cambodian friend who will co-sign for you. Some shops will sell it to you anyway for an extra charge. Alternatively, mobile phones are cheap and used ones easily available. For flexibility, opt for a pay as you go card.
Internet cafés are now popular across the country. Internet access is cheapest in Phnom Penh where rates continue to drop. Expect to pay around US$0.50 per hour, slightly more in Siem Reap and anything up to US$4 in the provinces. Wi-fi is common in the cities and cafes will offer free wi-fi to customers. Hotels provide them for a small fee. Traveling with your laptop? A universal A/C adaptor is handy. Unlimited broadband runs between $300-$400 a month.
International calling via cell phone is sketchy. Skype or Yahoo Messenger are best and most internet cafes will provide headsets for these calls. To use a landline, pick from prepaid cards for use from any phone or private booths offering calls via mobile phones. Or call from booths run by the Ministry of Post and Telecommunications or Camintel. Phone cards are available from U$$5-US$50. Use the pre-fix 007 for cheaper international rates.
Local calls are pretty cheap but province to province calls can be expensive.