Your question gives insufficient information regarding which call button you're referring to, and no information about which country you're in so I'll have to stab around a bit and hope to hit the spot..
If you're asking why doesn't Bluetooth voice dialling work, then the answer is simple and really annoying: The Hero does not support it. Some folks claim it's just because it's not yet been added to the Android operating system, but I was told by HTC and T-Mobile tech experts that the phone does not support it at the basic hardware level. I don't trust the latter argument as there's now a few folks who claim to have transferred the application into the Hero by copying the files from a different HTC model
(scroll down to post #44 onwards). I haven't been able to test this as I sent my Hero back when I discovered the lack of Bluetooth voice dialling. Android 2.0 *might* have it according to some, but the Android 2.0 website does not list it as a feature. HTC have not yet released their version of Android 2.0 yet so it's a case of wait 'n see.If you're with T-Mobile then the delay will be even longer as they'll need to tweak HTC's version of Google's Android 2.0 into a T-Mobile G2 variant <rolls eyes>. My understanding is that it will still be absent though.
If you're in North America, then the Hero already has voice dialling...sort of. Holding down the call key for a couple of seconds on the phone activates it. But you'll first need to touch the phone to wake it up, activate the voice dialling and then manually transfer the call to your headset. Utterly useless, and totally illegal in the many places where it's illegal to touch the phone whilst driving. On some networks the feature seems to be disabled and it does not work at all on UK/European models of the Hero.
There is another way to get voice dialling, but I'm unclear as to whether it will work via a Bluetooth headset: basically you install Google Voice
onto your phone which allows you to search your contacts by voice command. Currently it's available only to those who reside in the USA.
Hopefully I've answered your question in which case I'd ask that you return the favour by rating my answer. If not, then please add additional relevant details and I'll try to improve upon my answer for you.