I have a HTC Aria and love it----except the fact that I cant seem to make it voice dial consistently. I say a name and "mobile" or "home" and it almost always either messes up on the name, or the type of number to call. I use it on the road alot, and find myself so frustrated with the voice dial, that i just go ahead and break the law and dial manually.
An HTC support tech suggested using the search button (lower right hand side, looks like a magnifying glass). Press and hold search button for a second and a voice recording screen will appear. Speak slowly. I tried "call answering machine" and it started dialing without further buttons or commands. Still a bit clunky but it seems to work better than anything else I've tried. I still can't activate the voice command from the earpiece, so it's not really hands free.
Upgraded to firmware 2.2 and still have these problems. Can't find anything at HTC, AT&T or on internet the provides a solution--but lots of people have the problem. I've had this phone for about 6 months and every time I get in the car with it I want to throw it out the window.