The only way to restart the handsets is to perform a hard reset. To do so, hold down the sleep/wake and power buttons at the same time for 10 seconds. The phone should then start back up.
Some older iPhones are also affected, which suggests it's a problem with the software rather than the hardware. Those using iOS 9.0.1 are most affected.
iOS 9.0.1 was released last week to fix the bugs in iOS 9. This was followed by iOS 9.0.2 a couple of days ago. As has happened before, with its updates, Apple may have inadvertently introduced users to new bugs.
This isn't the only problem that users have reported. The home button has a tendency to get "really hot" according to some users. Others complain that the speaker distorts music and gives off "super loud" notifications, suggesting the speaker may be blown. Phone calls and ringtones can also come through the wrong speaker and sound at the wrong volume, according to 9to5Mac.
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Others say that 3D Touch isn't working, and that they can't open web links in Safari using a normal tap.
Previous iPhones have suffered more serious ailments. The iPhone 4 dropped signal when held a certain way, prompting Apple to issue free bumpers to anyone affected. And the iPhone 6 Plus bent a little too easily for some users' liking.