Based on default installation, your windows mobile might have a custom skin for dialer. Custom skins look nice...but are they fast? Well, at least on my HTC Diamond it seems that not. When pressing the end/begin call buttons there is some notable lag which is very annoying for me.

For example I want to make a call but in the very next second I change my mind and I want to stop calling. In this situation it might happen that while the phone is "trying" to stop calling, the other person already received a "bip". This is very bad so I've returned to the "old style": the default wm6 skin dialer seems to be very fast. Also the amount of consumed RAM decreased with about %10.

Below is the manual registry tweak:
-'HKEY_LOCAL_MACHINE / Security / Phone / Skin'
-change the value of 'Enabled' from 1 to 0.