I?ve the same problem . I live in Argentina an "Personal" is my signal provider, we have (5) Iphones 6 plus (my wife and sons) and after upgrading to IOS10 custom ringtones didn?t work no more in all our phones. They are still there, and Iphone can "play" them, but with an incoming call only works the "default ringtone", the same for all users, no matter what you do . I tried to asign them again, after re-installing IOS10, but nothing changes. The same for all my friends with iphone after upgrading.
Last weekend we went to "Iguaz? Falls", when we crossed the frontier with Brasil and the signal provider changed to "Vivos " (a brasilian cell phone company) and ...oh miracle! custom ringtones began to work in our iphones, but on sunday, when we returned to our country, they vanished again. Conclusion: IOS 10 is not compatible with all signal providers, many users have the same problem in USA and Europe, and Apple seems to ignore the problem. A few days ago i could downgrade to IOS .9.3.5 (now is not possible anymore) an all is normal now, but i can?t buy an iphone 7 if Apple don't bring a solution before . Anybody tried IOS 10.1? (I apologize for my english)