Q: iPhone 6 ringer occasionally goes silent until restart

Within the past few weeks I have 3 times noticed my iPhone 6 stops audio ringing for no reason.  Once this occurs, the phone still vibrates on an incoming call, but does not make a sound at all.  I can take the call fine and can even use speakerphone to hear the caller -- it's just that it doesn't ring other than vibration.

I have checked everything that I can think of including the mute switch (no effect), ringtone setting, ringer volume, accessibility (mute), etc.  The only thing that resolves the issue is to simply shut down the phone and then turn it back on.  It then rings normally again.  This has randomly happened 3 times in the past month, with no apparent rhyme or reason for when or why.

Although I normally have my phone in my pocket and therefore feel the vibration (this is how I recognized this problem), I hate that I might miss an important call if I'm at home and lay the phone down.

Any ideas?  Anyone else having this issue?

I plan on updating to iOS 9.2 this weekend so I'm hoping that the issue will just go away with the update.