Sony Xperia Z phone
 

I am on my 2nd Xperia Z and have the same annoying problem. Every once in a while (twice a week) the phone turns down the ringtone by itself. The first
I know is when I find I have missed calls. I go into settings and find the sound has set itself to zero.

Thanks in advance for any help/advice.

maybe you're turning it down somehow and not realising it? I have a Z3 and not experienced that.

Wish that was true span. Turning the volume button down doesn't turn down the volume control in settings.

When you are on home screen or in settings, or even an app that has no sound, the volume button controls the ring volume.