At first glance, it looks like you may have a spanning tree issue.  You have multiple physical ports in VLAN access VLAN 100 with portfast.  Typically portfast is used for end devices and not connections between network devices.  

Can you provide a diagram of you physical network to enable us to see an overview of what devices are connected to what in your environment.  The answer may be as simple as clearing the portfast setting off of your connections for vlan 100 to the firewall and other switches/routers.

Here is a good article on setting up VOIP on the 5548P switch:

en.community.dell.com/.../configuring-dell-powerconnect-55xx-series-switch-voice-vlan.aspx

I hope this helps,

- interface Gi1/0/24 is connect to firewall Internet gateway for Internet access purpose. - interface Gi1/0/21 is connect to point-to-point link for connection between DMZ subnet in firewall Internet gateway and DMZ subnet in data center via core switch. - interface Gi1/0/22 is connect to DMVPN router for connection of data, VoIP and VPN subnet with other branches. I also refer to the link above to setting up VoIP vlan but the article did not mention how to setting up tagged VoIP phone and untagged computer on same network port. Eg: network cable from switch connect to VoIP phone. Then another network cable of VoIP phone network port connect to computer. Izzit need to use switchport mode general instead of switchport mode trunk on the network port?

Here is a sample config:

****VLAN 10 is configure for voice****

****VLAN 200 is configure for data. ****

****CoS (802.1p) of 4 is use for the the voice****

Console(config)# voice vlan

Console(config)# interface ethernet 1/g10-1/g15

Console(config-if)# switchport mode general

Console(config-if)# switchport general allowed vlan add 10 tagged

Console(config-if)# switchport general allowed vlan add 200 untagged

Console(config-if)# switchport general pvid 200

Console(config-if)# switchport general allowed vlan remove 1

Console(config-if)# voice vlan 10

Console(config-if)# voice vlan dot1p 4

Console(config-if)# voice vlan data priority untrust

Console(config-if)# end

The PVID command is what tags the incoming traffic that has no voice mac address.

Page 615 of the owners manual goes over the Voice VLAN commands.

support.dell.com/.../cli_en.zip

Page 384 of this manual also takes us step by step on creating a Voice VLAN.

support.dell.com/.../ucg_en.zip