New day, new challenge !
We faced an issue where users who use/used the Skype for Business mobile client were not able to get simultaneous ringing working…
Even they disconnect and uninstall the application, simultaneous ringing was no more working for their mobile device set in AD but it was working with another number.
Then I launched the lync debug tool and saw that it was trying to reach the sip address email@example.com (the phone number to reach was on an iPhone)
So I tried to test the push notification with this command on the front-end server :
Test-CsMcxPushNotification -AccessEdgeFqdn EdgeServerFQDN
So I followed the article to configure push notifications : https://technet.microsoft.com/en-us/library/hh690047%28v=ocs.15%29.aspx?f=255&MSPPError=-2147217396
Then simultaneous ringing start to work again for those users who were using the mobile app.