Well...it's not clear.
The manual just says:
"A voice VLAN is configured for voice traffic. After assigning the ports that connect to voice devices to a voice VLAN, the system automatically modifies quality of service (QoS) parameters for voice traffic, to improve the transmission priority of voice traffic and ensure voice quality."
But no details about what QoS parameters are then altered by the Switch...and that automatically into the statement "...the system automatically modifies quality of service (QoS) parameters for voice traffic..." and is a little bit misterious (maybe a diff about Switch config before/after activating Voice VLAN could shed some light on that).
I can't believe - but I could be totally wrong - that simply enabling a Voice VLAN then automatically let the Switch to create a sort of pre-configured QoS Policy with Class, Traffic Behaviour and Policy on all Voice VLAN ports (as should be done following User Guide instructions).