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Here is the topology that I have:
Phone => (access port) L2 Switch (trunk) => Router
My goal here is to add an EF tag to the packets coming from my phones. Basically, if a packet comes in being in Vlan 30, I want to modify the L3 header and add DSCP EF.
Would that be possible to do that on my L2 switch, or can I only modify the CoS tag ? My switch is a C3750
More than likely, packets from the phone have the correct DSCP values: EF for the data, and something like AF31 for the control traffic (you really do not want the control traffic marked as EF). Your switch will change the traffic to BE unless you use the mls qos trust dscp interface command.
Yes, you can set DSCP values on the packets with that switch. It can get complex, setting up ACLs, class maps, policy maps, etc., and you can configure VLAN-based QoS (mls qos vlan-based).
Cisco maintains many documents about this. For example:
BE when QoS is enabled unless you trust DSCP on the interface. I believe that was they way the switches worked until the 3800 series. We have comprehensive QoS, and we have eliminated those switches, but I am familiar with how they worked, and I still have the configurations. Specifically for VoIP, we simply trusted what the phones provided, but we had all the other configurations for various other traffic.