Thanks. I too was getting a ring tone whilst calling from my landline, but not from my other cellphone!
The first time it happened was a month ago. After fiddling around unsuccessfully with various ringtones, I desperately tried 'Settings-General-Reset' with some trepidation, not knowing what would result! Luckily it worked, and my alerts resumed.
However, my incoming calls and messages again stopped giving an alert earlier today. That's when the (relatively unfamiliar for me) idea of checking on the net occured to me.
Fortunately! Because after reading your post I checked, and discovered tht my setting too had also changed inexplicably to 'Settings-Do Not Disturb-Allow Calls From-Favourites'.
Once I reset 'Favourites' to 'Everyone', the alerts have resumed.
So thanks again.
Just to be on the safe side, I also checked 'Settings-Accessibility-Assistive Touch-On' and tapped on the 'Black Square with White Circle' icon, which give me the first set of menus with 'Device' as one of the options.
Tapping 'Device' opened a new set of menus where I confirmed that the setting for Mute/Unmute was correctly set to 'Unmute', indicated by a bell with a line across it, with the option to 'Mute' available below the icon.
Of course I retraced my steps carefully and switched 'Assistive Touch' to 'Off'.
I suspect that it is a bug in the OS that triggers an unintended change in the settings on some arcane inputs! Hope Apple is aware and actively attending to it.
Bhup