Calling TO the US FROM HK, it doesn't matter if you have a tri-band phone. If your phone works in HK, you can get a SIM card for it with good outgoing rate.
Currently the best SIM card with IDD is the
Peoples' Stored Value IDD & Roaming SIM Card. This gives you a rate of $0.25 HKD per minute to call several countries, including the US. It also is a roaming card. The downside is, local calls are also $0.25 per minute, which is expensive for local. I use this as my long distance SIM card, and a Sunday plan for local calling (Sunday's IDD rates are ridiculous compared to this).