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.
On the PRESENCE™ the voice dial works only with the last paired 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" (ie: the last phone that was paired).
Also if using a BTD 800 and it was paired after the PRESENCE™ was paired to a mobile phone the BTD 800 is now the device that was paired second hence it is the one that the PRESENCE it trying to use for voice dialling. Typically voice dialling via BTD 800 to softphones is not implemented by the softphone manufacturers and hence voice dial is not an option via the BTD 800.
In a situation where the device that will be used with voice dialling is not in the correct sequence (ie: it is not the last device paired) you can remove existing pairings and re-pair the devices in the sequence that fits with what voice prompt needs. For example pair the PRESENCE™ to the BTD 800 and then pair to the mobile phone so that the mobile phone is the second device paired and hence will receive the voice dialling prompts.