OK, maybe it would be useful to know this:
802.1q is the ethernet standard for VLAN tagging. The 802.1q frame format contains both a VLAN ID *and* the 3-bit QoS class of service.
So, if you assign a VLAN to a port as a tagged VLAN, then frames in the VLAN will be passed with the class of service included.
So long as the phones are correctly configured to assign "5" to the voice packets, and so long as the voice VLAN is tagged everywhere it goes round the network, then the voice packets will be marked with the "5" wherever they go. You then have to trust that the default QoS config on each switch deals with packets marked "5" correctly.