What is: a voice VLAN port?
A voice VLAN port is an access port attached to a Cisco IP Phone, configured to use one VLAN for voice traffic and another VLAN for data traffic from a device attached to the phone.
This feature allows the switch to use CDP for sending configuration options regarding COS, Voice VLAN and data VLAN to the IP Phone. This feature is disabled by default and the IP Phone sends both voice and data traffic in the default VLAN (VLAN 0).
you can use CISCO documentation for more details about this feature.