cmonex is absolutely right. Dual-band means it works on two frequency bands. The two frequency bands in Europe are the same as those in Asia, but are different from the bands in the Americas. Since the PHH was marketed in Europe, it would only support the two frequency bands in Europe.
A tri-band device would support the two local bands, plus the higher band from the other region, so it would work with at least that band when you travel. I personally got my cellphone from Asia, and it had to be tri-band.
There are now quad-band phones too, which obviously would support both bands in both regions