Theres no way to set the phone to not enter lowpower mode (standby).

When youre making the call, are you sure youre not covering the devices proximity sensor (the one that turns the screen off when you hold your phone to your ear to prevent mistaken commands to the screen and blacks out the standby clock when you pocket your phone?

"Pressing 2 times in a row to exit the phone from standby".. do you mean double tapping or do you mean doublepressing the powerkey?