When I had my Peoples prepaid SIM card that charges the same for local HK or US calls, the local charge (HK to HK)) per minute ($.25/minute) is more than the plain local, non IDD calling card. However, there was a steep roaming charge. If the phone was taken into China, calling from China using the same SIM card was something like HK$12 per minute. (not sure about the precise $, but it was very high)

I suggest using another kind of IDD roaming prepaid SIM card if you have to take the phone to China or Singapore.