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That is tagged as a 'solution' - thats not a solution Rickard! Its a copout! Tell Android your customers are demanding the feature be re-instated! The customers that ultimately keep you employed Rickard!
Hahahah 
This is so funny 
So - at first you (Sony) has created a crappy dual SIM product (my D6633 / Z3 Dual) in which:
- dual sim is not working at all - any combination of "phone call on SIM X and internet on SIM Y" causes that there is no internet
This is just a big laugh at dual SIM philosophy
and at specs presented in manual.
- a popup with choice of which SIM should be used fo call is often not showing
- device often drops cell network connection saying "network unavailable" and then it shows full signal power after 5 seconds, but then disconnects my call again after half a minute and so on, despite I am just sitting at my desk in the office.
This was happening on 4.4, 5.0 and 5.1. So, I have sent this "device" to your service center. And I got it back with Android 6.0. And now you are informing me, that "in Android 6, it's no longer possible to set ringtone to your second SIM card. This is a limitation on platform level from our platform supplier"? Is this a joke? At first Sony has sold broken phone, and then it repairs it in such way that it misses most elementary setting because of "limitation on platform level from our platform supplier"? Then change your supplier! Why other vendors can have Andoid 6.0 dual SIMs and still can allow for choosing separate ringtones and Sony can't?
How dare you to say such nonsense? Then maybe you are going to invent Sony Z7, which will be unable to support cell phone network because of "limitation on platform level from our platform supplier" and it will work only with Wi-Fi and USB cable? How about Z8, which will be able to communicate only via sloooow MTP through USB and will be used only for making photos and playing music? You can also put on the market Z8 Dual. Which will support dual SIM cards just for a joke, because there will be no setting for ringtone at all, as it would work only communicate with poutside world via USB... Pathetic. My friend has right saying "Buy OnePlus2 - more cores, more RAM, same other specs, less money. And a dual SIM too." And I was so stupid to not listen to him...
Still no fix? Will this feature be available in android 7?
Is there someone from Sony reading?...
Up... 
Hope this will be really solved with Android 7 in december otherwise, bye bye Sony!!!
Nos that Android Nougat si arriving, does the dual SIM ringtone is back?...