It looks like I have found the reason:
On Microsoft website it says that the HP's Ipaq and WM5 has bluetooth compatibility problem.
Here is what is says on Microsoft website:
"HP IPAQ
The audio routing of announcements and the initiation of Voice Command interactions will not work properly on this device due to the Bluetooth implementation."
Link:
Voice Command for Windows Mobile 5.0
So if anyone knows a way to go around this issue with IPAQ please share !