I already answered this in the htc forum

Hi All

I'm a Cingular (AT&T now) user currently visiting Singapore.
I found a cool smart phone made by HTC and I want to bring one of them.
There are two models I'm interested in.
One is tri-band with 900, 1800, 1900 frequencies. It seems Cingular uses 850 and 1900 freqeuncies, so I'm not sure whether to buy it or not.

Second one is quad-band with 850, 900, 1800, 1900 frequencies so I guess it's more compatible with Cingular than the first one.

But my real question to ask you guys is how I can unlock the phone? Since HTC hasn't started selling the phones in the US market, I don't think I can get help from Cingular customer service. I sent e-mail to HTC asking the same question, but they haven't replied yet.
Can I unlock a phone by simply pressing secret code using the key pad? Or do I have to physically change a chip inside the phone?

Anybody know the answer?