HTC abbrev. stands for High Tech Computer Corp of Taiwan.
You can find them on the web at
HTC actually is the OEM manufacturer for these phones.
A bit of trivia is that they also manufacture the iPaqs for HP (formerly Compaq).
T-Mobile and O2 took on the branding themselves,
but AT&T had the forethought to outsource a portion of that activity to a name that would be recognized by consumers... Siemens. So Siemens doesn't have anything to do with manufacturing them... it's a strange business.