Proposal: urgency indicator feature for phones

I’d like to propose a new technology or feature to be used in cell phones (or maybe Internet phones, if they become widespread). Often, when someone calls, whether or not I answer the call depends on the priority of the call (as judged by the caller). This situation especially arises when I’m in class. But there’s no way to tell how urgent it is, aside from forcing the caller to keep calling multiple times. That takes more time and isn’t universally known or intuitive.

What I’d like is a feature to choose a marker to communicate your call’s urgency, at time of dialing, so that it appears on the receiver’s screen (like Caller ID) without necesitating opening the phone. There could be 2 extra levels: urgent (if possible, please answer now), and low priority (just calling to casually chat, don’t answer if you’re in class). There’s also the default “normal” mode for phones that don’t support it or don’t specify a priority. This should be easy to implement in Internet phones. For cell phones, it should be technically feasible. But I’m not sure if it could be implemented completely in software, or requires hardware modification. It might get fairly technical. As far as the user interface, how about adding another number to the end of a phone number– or pressing a certain button before calling.