Teru,
I've heard that Cisco phones can act as a mini-switch, taking an ethernet connection from the network, and providing an ethernet connection to the desktop. This allows both voice and data to ride the same network connection. The way that the phone differentiates between voice and data is by accepting a trunk from the network, switching data from the data VLAN to the data port, and keeping traffic in the voice VLAN for itself. This would only be necessary if your VOIP design includes separate network lines for voice and data.
HTH
Mark