Voice VLAN with 5448

While trying to understand the Voice VLAN feature I had some questions coming up.
Setup: The VoIP phones are connected to two PowerConnect 5448 floor switches. These PowerConnects are connected to another PowerConnect 5448 in the server room to which the PBX is connected.
Goal: I want to achieve the prioritization of our VoIP traffic.

What I tried:
- Created a VLAN with ID 10, name "Voice"
- Enabled Voice VLAN globally using VID 10, CoS 5
- Enabled "switchport mode trunk" on the trunk ports
- Enable "switchport mode general" on all ports with VoIP phones and on the port with the PBX
- Enabled Voice VLAN on the edge ports

I did some testing and sniffing, but the sniffer didn't show any tagged packets (I made sure that the ethernet driver does not strip VLAN tags).
So I assume that all traffic is still sent in VLAN 1.

My questions:
- Do I need to enable Voice VLAN on the trunk ports?
- Do I need to enable tagging on the trunk ports or is this done automatically?
- Do I need to enable LLDP-MED?
- Do the VoIP phones need to send tagged packets?
- Anything else I'm missing?

I would be glad if someone could walk me through the configuration of Voice VLAN.
Thanks for any input.