Will my cellphone be usable in Cambodia? Well the short answer is that if it’s reasonably new it’ll work unless it’s been locked into a very unpopular format. Best thing is to check with your service provider. LTE is becoming available in Cambodia but as yet it’s expensive and not widely supported.
Another option is that you could pick up a cheap phone in Phnom Penh for $60 or $70, use your expensive phone for taking pictures. Get a local sim card for making local calls and mobile internet.
Leave your phone at home and setup a call/message forwarding system to your Skype or equivalent service.
Don’t leave it unattended or on a bar top, or restaurant table. You’d be amazed at how little you notice going on just next to you. Install a security program such as Cerberus for Android or Find My iDevice to help secure your phone in the chance it gets lost/stolen.
Get A local SIM
Yes it’s easy enough to get a local SIM card for your phone. There is a tourist card available but it’s more expensive to use than the local SIM but it saves you having to ask someone to get you one.