Tell the switch to trust CoS markings from a Cisco IP phone on the access port. Cisco IP phones use 802.1q tags, these .1q tags contain the CoS value, to mark voice traffic at layer 2. When it's forwarded upstream, the DSCP value is trusted (on the uplink port) and unchanged, but the .1q tag (and with it the CoS value) is stripped off by the upstream switch when received over the trunk.