Xperia M2 Ring Volume Drops
I believe I have tracked this to an app.

Three In Touch (version 3.4.5) is a pre-installed VOIP app from the service provider (Three) that uses Wi-Fi to route calls if there is limited signal.  After disabling this app the volume drop went away and re-enabling it has brought it back.

This is unfortunate as the app is otherwise very useful.