I certainly agree that those QoS settings are intimidating.
What is it that you need to achieve?
(and are you sure the calls aren't being dropped due to a firewall misconfiguration rather than a QoS one?)
I am assuming you need to target either a UDP port or maybe the IP address of your linksys box to be higher priority than other traffic?
The GUI isn't clear to me but it looks like you are trying to add a traffic class on this screen, which is the details in the bottom (presumably you only fill in the relevant bits) and then presumably you have to give it a name in the top section and set something sensible in the two drop-downs in the top bit. My guess is that you probably don't want to fiddle with the middle bit - the Linksys box probably already marks VoIP traffic with whatever DSCP mark it sees fit anyway, so you probably don't want to change it.
If you add a rule like this and hit apply does the screen then change... or does the rule appear in another screen to be used, or is this all there is?