During the installation of this mod, you can choose between "enable popups" or "disable popups".
It allows you to enable/disable the on screen notification for missed calls and new messages.
Restart the phone after the installation.
I tested it on my Belle FP2 CFW (v2.2.1) on the Nokia 808.
But it should work on most other Symbian devices aswell...
The file is unsigned, your phone needs to be hacked for this mod.
(All my CFW's are hacked of course)
If you try it on any other device than on the Nokia 808 (FP2), please drop a comment to confirm its working for you too!
You can download this mod in the tools/download section on my blog.