Look
here at this thread
and this is what you want..........
Repeat Notifications--Turn them on
This will enable the "Repeat" option within Sounds and Notifications for Phone: Missed Call.
\HKCU\ControlPanel\Notifications\{A877D660-239C-47a7-9304-0D347F580408}
Options
Value data: 1073741845
Decimal
Changing this registry setting enables the Repeat option as well as "Flash Light for 15 minutes". You can make changes to any of the options, as needed and the Repeat should stay enabled.
Alternatively, you can do a quick FIND within your registry for "missed call" and this should help you to quickly locate the proper key.