i have an iphone 4 running the latest ios, 6.1.3 and i have noticed recently that sometimes, when the phone is locked and i have a missed call on the lock screen, then the phone rings and i answer another call from a different contact, sometimes, after ending the call, the missed call notification disappears from the lock screen. this doesn't happen all the time, just sometimes. i know that if i receive a call from a different contact that this whom i have the missed call from, the missed call is supposed to remain on the lock screen until i unlock the phone. why is that happening and how to solve it? i noticed that this just started happening after the last software update.