You need a program that allows you to change the registry settings. Total Commander is what I used. Then go into
\\plugins, find the "registry" folder under that, then browse down the path until you reach
HKLM\Software\HTC\PHONE\PhoneExtendSetting
In that folder you will find the key "EndCallTime"
Open that key and you should see a box with "3000" in it. Change it to 500, 250 or whatever value you desire. 500 is 1/2 second, 250 is 1/4 second &c. I set mine to 250. Then hit the OK button. You may have to do a soft reset for it to refresh.