Share your experience!
I believe this happened after I upgraded the phone to the latest version.
At first I did not have i ringtone at all, but after I fixed that the phone only plays the original tone. I have the same problem with the message tone. I even tried to put a specific tone to a specific contact. No difference 
Here is a screenshot of the sound options where you can see that I have changed the ringtone to "Trap Android" and the message tone to "Ping":
Solved! Go to Solution.
I used Zedge (the app) where I downloaded the tones. The Z3 let me choose the tones and I have used both the app and the tones all the way, but I could not find them on the phone.
So somehow they have been deleted from the phone. Downloading the tones again and it worked fine.
Now it works fine!
Thanks for the help!
Since upgrading the ringtones I set for particular callers from my music collection on my SD card do not work.
I used Zedge (the app) where I downloaded the tones. The Z3 let me choose the tones and I have used both the app and the tones all the way, but I could not find them on the phone.
So somehow they have been deleted from the phone. Downloading the tones again and it worked fine.
Now it works fine!
Thanks for the help!
The same problem: my own ringtone setting is forgotten. My own ringtone is on internal SD card, media/audio/Ringtones. After I exit the sound settings, and wait several minutes, the ringtone is forgotten and the phone plays the default sound when ringing. When I re-enter the sound settings, the ringtone appears as set to "Unknown ringtone".
Entering the safe mode, setting the ringtone to mine, and going back to normal mode does not help.