phone goes into lock mode while connected to bluetooth

I get into my car and pair my iPhone 4s with a bluetooth car speaker.  Works fine.  Set off on a drive.

Then sometime later the iPhone goes into lock mode, and the bluetooth, while it is still paired, cannot control iPhone functions.  Getting it to work again requires taking the iPhone out, sliding the unlock, and entering the code.  Not convenient or safe while I'm driving.

I have my iPhone set to go to lock mode after 5 minutes of inactivity.  It takes longer than 5 minutes to lock when it is connected via bluetooth.

I realize I could set my phone to "never lock," but I want the lock function active in case I lose the phone.  It would be a nuisance to go through the settings and turn lock mode on and off each time I got into or out of my car.

Shouldn't the iPhone not go into lock mode when it's bluetooth connected?  It's in use, not idle then. 

If it matters, I'm using a Jabra Freeway as the car speaker.  Same thing happened with an older Motorola I used to use.

How do the rest of you handle this?  Is there a setting I'm missing?