CallScreenOff for Android
The idea is as follows:
1. You accept/make a call or hang up: goto 3.
2. The screen unlocks and you are in a call: goto 3.
3. CallScreenOff checks if you have a BT headset connected: if yes goto 4, else do nothing
4. CallScreenOff uses the proximity sensor to determine if your device is in your pocket: if yes goto 5, else do nothing
5. Use Device Manager to lock your screen
6. Done!
More information:
http://www.rejh.nl/callscreenoff/