As an update to this post – http://sysadminman.net/blog/2010/updating-o2-htc-desire-to-android-2-2-froyo-1340
It looks like the Froyo ROM I initially installed did not include voice dialling. I have now installed LeeDrOiD which does include voice dialling over bluetooth that works with my Plantronics 975. I install LeeDrOiD v1.9 which can be found here – http://forum.xda-developers.com/showthread.php?t=724123
You need to ensure your wireless drivers are up to date before installing the LeeDrOiD ROM. Check out the link at the top of this post on how to do that.
As I had already ‘rooted’ my Destire installing the new ROM was a simple case of downloading via WiFI on my Desire, rebooting in to recovery mode and then selecting the ZIP file to flash. I did not wipe this time as I already had Froyo on there, however wiping is recoomended.
Initial thoughts are that voice dialling doesn’t seem as acurate as on the iPhone IOS4, but it’s early days yet. There are also a lot of other tools/utils included with the LeeDrOiD ROM.