Tom is right. Feature codes are the way to do it.
To make it as easy as possible for the user, configure two Speed Dial buttons (or whatever they are called on the Linksys)
One you configure with the digits: ##20*#, this will enable call forwarding unconditional (all calls) to the voicemail of the user.
The second button use the digits: ##23, this will deactivate
The reason this works is because SwyxWare enable the call forwardings for the users serverside and not in the client itself. This means that when you have enable the call forwarding, you can actually unplug the phone if you want and it will still work!!