The issue occurs when trying to reply to a missed call notification from the lockscreen while the network is searching for signal. In the demo video, iPhone Islam team replicated the issue by removing SIM card, and waiting until searching shows up in the top left corner of the screen, then swiping the missed call notification on the lockscreen to reply.
This security flaw also gives access to your phone`s contacts list, favourites, recent calls as well as the ability to send emails and make calls.
This is a major flaw considering it gives access to your phone`s private information which you originally placed the passcode to protect. Hopefully Apple will fix it in future updates.