A further thought about unsubscribing from call queues at the client end. This should ideally be on a call queue by call queue basis. For e.g. I might be part of the externally facing technical support call queue but I am also a first aider and the company could set up a call queue for all first aiders - essentially an internal 911. If I am at a meeting I would want to unsubscribe from the tech support call queue but would remain part of the internal 911 call queue
One step further would be to manage it by configurable profiles - there is not much point in me getting the internal 911 call if I am at home 20 miles away but I might still need to be contactable so don't want to log off. So, in meeting, off site, off duty, on holiday, profiles with different call queues subscribed to under the different profiles would be great but you are obviously getting into quite sophisticated use of the system at that point and end users that know what they are doing. However, I can see that for a major industrial facility or a hospital or something of that nature it could be a pretty efficient way of managing different incident response groups - where the call "Queue" is really being used to call 20 different phones at once and you don't care who picks up just as long as a first aider, or cardiac specialist or whatever picks up the phone. You could even have it so that if no one picks up after 20 rings the call gets routed to the site Public Address