If you have a Windows Mobile 5.0 device prior to build 15097.3.0.0, you will not be able to receive spoken caller ID notifications through the Bluetooth headset. Windows Mobile with build number the same or higher than build 15097.3.0.0 includes support for Inband ringtone which is the mechanism that Voice Command uses for announcing caller ID information while the phone is ringing.
To view the build number for your device, you will first need to know if you have a Windows Mobile device with Pocket PC or smartphone software. An easy way to tell is to look at the Today screen and find the Windows Mobile flag. If it is in the top left, you have a Pocket PC. If it is in the bottom left, you have a smartphone.
For Pocket PC or Pocket PC Phone device:
Start Menu->Settings->System(tab)->About(icon)
For Smartphone based device:
Home->Start Menu->Settings->About