My Siem Reap hotel suggested Mobitel but upon arrival in phnom penh could only find smart, beeline and met done kiosks. So $2 for SIM card, $5 for voice, and $5 for 2gb data and Mobitel set up on backup 3G iPhone running 3G speed.
First two days good service. Lots of local calls to confirm appointments, get directions for driver and of course accessing emails. Hope it works in SR....