Hello,
Resolution: 
Turn on Hands Free Mode.  Settings (the gear) --> Controls
         
 
Look for a steering wheel and turn it on.  
Go into that Hands free menu and turn off/on the settings appropriate to you.  
Drawback: You must lock your screen manually while on calls.
I assume you have hands free mode...
I am an android developer: 
Alternatively, if you or others still can not get past this issue please msg me and I will put together an app that solves this over the weekend.
  App function would be: 
  -whenever you are on a call the code/app will be invoked to start and run in the  back ground only for the duration of the call.
  - when you press the home button or lock screen button, the password screen / unlock screen should show. 
Anyways, if there is enough interest i'll do it.
ahsan.noorudin.ashraf at gmail.com
Hope this posts helps!!