I have faced these issues across various builds of Windows Mobile 6.5 such as 23004 / 23016 / 23085 to the extent I often went back to the stock version of 6.1 because I couldn't afford to not know who had called me.
I would like to credit Rayraven from the Winmo forum for helping me resolve this. By adding a patch file to the windows startup this solves the issue. The file is called ks20.exe as it would built for that phone where users faced the same problem.
The original file is posted on Xda, I have however mirrored it here for ease of use. The program on running will consume little bit of system resources and will only be visible in the task manager process view as seen below.
The file is located here
http://www.mediafire.com/?cz7bevlgf7qxu6s
Put it in the following location
My Device --> Windows --> Startup
The article about flashing to windows mobile 6.5 is located here
http://p320.blogspot.com/2009/11/asus-p320-windows-mobile-65-rom-build.html
The main page about this blog is available here
http://p320.blogspot.com/2008/11/asus-p320-how-what-where.html
