To use the voice dialling function, hold the multifunction button for one second. After hearing the voice prompt "voice dial", then say your voice command.
Please note that this feature only allows access to the voice functions of the end device, this is not a feature of the headset itself. For example, when used with an iPhone, voice dial will activate Siri, and when used with a Windows smartphone, it will activate Cortana. If your smartphone does not support voice commands, voice dial will not work.
The voice dial feature also only works with the last paired device (the highest priority device). So if two mobile phones are connected at the same time then voice dial will only work with what the voice prompt refers to as "Phone 2" (i.e. the last phone that was paired). For more detailed information on setting the priority of your paired devices, please see the dedicated entry.