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That's why I hate updates, one feature added and 10 new problems.
I also use SIM1 for personal calls and SIM2 for work and this feature is important!
I miss Kitkat... 
Hi, I'm a french user of XPeria Z5 Dual.
I asked on the french Facebook page and I've been answered that this function will be readded on the next update (about may 2016 in France), can you confirm?
Thanks.
i think we have to act as a community. Lets all send emails to the same email address complaining about this lost feature. Its important to send emails to the same Sony address. Otherwise if everyone sends to their sony's country email adresses sony may not be able to see the volume of complaints. Please someone that has already contacted Sony about this issue, tell me th email address they used.
No solution yet. Common Sony. How much time this ones gonna take? Hardly some job this is Sony SW developers. Common.
even i am facing same problems it is bug of marshmallow and samsung is fixing bugs faster by providing many updates and sony developers are not doing anything
@Ram786 wrote:even i am facing same problems it is bug of marshmallow and samsung is fixing bugs faster by providing many updates and sony developers are not doing anything
Can't agree more.
I contacted Sony and got the same responce and little help.
I managed to downgrade the firmware on the phone back to lolliepop. I'm up and working fine again with dual ringtone. 
To do this you need to put your phone into Development mode.
See here
http://androidfact.com/how-to-downgrade-android-firmware-and-versions/
Then find your firmware version you need to downgrade to.
Download the Flashing tool. (ignore the bit about creating your own ftf) select the file and flash the phone.
Instructions below
Be aware that your backup will not transfer your apps.
Good Luck hope it helps
@Rickard wrote:I've received more information on this and 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 and this feature will not be available on our products on Android 6.
Rickard, if possible just try and get your hands on a cheap chinese dual SIM mobile with MM, and you will find this very setting on that phone. I can give you names of lot many phones having DUA SIM and running MM like Lenovo Note4, Leeco 1S, Xaomi (any model with dual SIM), etc.. These are mid range phones lets talk about a comparative competitior of Sony as LG(G3,G4 both running MM with dual sim) and Samsung(Note 5 dual sim, S7 Edge with 2 sims) even these high end phones running on MM have this very feature.
SO MY request TO YOU IS PLEASE DON'T LABEL YOUR INCOMPETANCY AS PLATFORM'S limitations. Accept the fact that Sony developers are not capable enough.
Also
After too many complaints, I got a call from Sony India yesterday. The fellow wasn't ready to accept all these bugs and was more keen on asking me to give my phone to service centre. Then I had to catch his ear the other way round and after many arguements he accepted.
And as a matter of fact he told me that he will take down this problem(bug) as feedback and will try to improve. CAN YOU BELEIVE IT???????? A dual SIM phone without DUAL RINGTONE??????????? Are you kidding?
@Rickard wrote:I've received more information on this and 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 and this feature will not be available on our products on Android 6.
And one more thing Rickard, Please don't mark any thread as solved untill you provide the users with a concrete working solution. Nothing is solved just as you mark them as solved. Solution means thing should start working after applying that solution.