I've been reviewing information on GSM in Singapore and note that they offer a automatic, free service called "Local Direct Dial" or "Local Direct Call" for calls to foreign roamers in Singapore. It looks like if a roamer is on a specific network, another user (roamer or local) on the same network can call him by dialing "#0"+CC+AC+NNNN. I'm not sure exactly how the call gets billed. Does the calling party pay just the cost of a local call or does the roaming party get charged for incoming calls?
Here are the LDD/LDC pages for StarHub & SingTel
StarHub LDC page
SingTel LDD page