Some say they have since upgraded to iOS 9.0.2 and are waiting to see if the problem reoccurs.
Heresone post from the MacRumorsforum that details the new problem:
Rose Gold iPhone 6s running IOS 9.0.1.
The screen was off, and the phone rang.
The screen remained dark, but the ringing kept going, but couldnt be answered.
Just pulled phone out of pocket and its dead.
iPhone 6s 128GB SG iOS 9.0.1
Havent had it with 9.0.2, yet.
But there are still likely to be bugs hiding around that need to be taken care of.
source: www.techworm.net