I am able to calibrate each of the esc's individually by connecting them individually to reciever itself directly.
But when I try to calibrate all four at once its not happening.
I am doing the exact process for calibration as mentioned everywhere, like keep throttle up and connect the battery and so ...!
But I unable to make them to calibration mode.

I have done everything perfectly.
Right from the assembly to the calibration.
I have uploaded firmware Arducoptor v3.2.1 through misson planner and done every calibration work correctly.
But still when it comes esc calibration part its not getting done.
I am using Fs ct6b as transmitter.

I had no clue what might be the reason.
I have searched everywhere for the solution and haven't found any way, finally lost my patience and landed here hoping to find some help...!

am doing the exact process for calibration as mentioned everywhere, like keep throttle up and connect the battery and so ...!

    After disconnecting & connecting the battery, are you getting a flashing Red & Blue led ?
Is the APM recieving radio signals.?
Radio calibration done ?

Radio calibration is done and APM is able to recieve the radio signals. And on APM blue and red lights are flashing continously.

after reducing the throttle to zero , do you get the musical tones from the esc ?

No its not comming,
Thats the main problem.

Even when I keep the throttle at max and connect the battery the to tones heard is also same as just the normal to tone that we hear every time we connect battery. And when we make it down to zero there is no sound.