How to lock iPhone5 while on a call without hanging up on the person?

I know people get confused so let me say I'm NOT TALKING ABOUT THE SENSOR
- what I mean is lets say I'm on IG or something and someone calls me and I want to lock the phone to save battery because you're wasting twice as much battery when you're on the phone and it's unlocked and open, if I were to press the power button it hangs up the phone call which is annoying and it doesn't autolock while on a call so how the heck do I lock it without hanging up on a person?

Not sure what you mean by LOCK and what you expect to happen when it locks

LOCK = can't access the facilities
eg the phone is (screen) locked and you can still answer the phone (incoming call)

It is in exactly the same (phone) state when you have it unlocked and Start a call.

The 2 power hungry parts are
- Making /receiving a call, and 4G uses more than 3G which uses more than 2G
So to save power set teh network to use 2G only! Or 3G/2G only

- The screen lighting,
so set the Screen brightness lower (makes a big differnce) - as low as is useful still.

The screen turns off in a call anyway,
- When backlight timer times out
- when you hold the phone next to your face (body sensor detects face and screen locks to prevent accidental button pressing and turns of light to save power).

The apps continue running Locked or unlocked.
but would suspend if a call came in / or was made.

So what extra do expect to happen?