by default, IP Phone`s interface that connect to upstream SW is working on native vlan mode, all of traffic (include voice from itself and date from PC) are untag. people in order to distinguish between voice and data, so using different vlan to transfer voice and date. here trunk is used. for example:
switchport access vlan 100 (for date)
switchport trunk native vlan 100
switchport voice vlan 200 (for voice)
switchport trunk enc dot1q (to direct IP Phone using trunk)
exclude 4000 6000 serial, they are using DTP to auto create trunk with IP Phone.