Most people get confused with the setting on thier pbx. everyone generally has thier voice on its own seperate vlan and when they see that setting, they think "oh, i probably need to turn this on as well". But generally you already have the port on your ethernet switch assigned as an access port (or pvid)for your voice vlan. This keeps you from having to turn that tag on in the system. However, if you were ever in a very odd situation where you have more than 1 device plugged into that switchport along with your 3300, then that switchport would become a hybrid or trunk port allowing tagged traffic for your voice vlan and what ever other vlans you wanted to allow on that port. At that point, the tagged setting on the 3300 would be effective to use.