When numbers not listed in "No screening" option of mobile calls then light of the mobile shuts on and off due its "rhythm ringtone" option. The light sensor circuit activates and deactivates due to fluctuating light. The time delay circuit is powered through this light circuit. But it does not get enough time do further activate the dual timer circuit as it is set to 20 seconds. this is how it behaves
[video=dailymotion;x17r3bm]http://www.dailymotion.com/video/x17r3bm_1[/video]
therefore numbers not listed in no screening option cannot open it as they fail to activate the delay timer circuit for 20 seconds