In order voice dialing to work through a Bluetooth device, you must first have paired your device to a compatible Bluetooth headset. For more information on pairing Bluetooth devices, see the Bluetooth category.
To use voice dialing with a compatible Bluetooth device, follow the instructions on the How do I quickly call a contact using voice command? FAQ.
You can use keywords like "call <contact name> <contact type>". For example, "call John Smith home" will call John Smith's home number.