Opportunity: Many a time one may face the situation where in
             one is busy for a couple of hours attending a
             business meeting, nature's call ...etc

             In the above scenario's it may not be possible for
             one to attend or pickup call's on one's mobile devices,
             while most devices on the market today offer an option
             of screen list.

Design specifications: In lieu of screen list  ....in addition to
                       the silent mode  ...give the user to set
                       the phone say a busy mode or various mode's
                       say kind of various profile's or customizations
                       and attach or tag different kind of screen 
                       list of incomming call's to these mode's or
                       profiles

                       also ability specific or set how long the 
                       device is going to be in such mode or update
                       the service provider's database that unable
                       to take up call's for specific duration with
                       a customized message (ideally to mode or profile).

                       The user's calling up the device would be 
                       better adviced to make a call.

Note: The above problem statement having been encountered in various scenarios
      and detailed in various 'Proof of concepts' as mentioned in 
      'http://uk.geocities.com/ravivenkatus/projects.pdf' ....apply appropriate
      'use-case' modeling, rationalize and arrive at a workable and feasible 
       solution both commercially and techinically viable.