I own a Sony Ericsson T630 mobile phone. To turn it off I have to press and hold the power button for about one second. Then I hear the turn off sound and I know the phone will be shut down. To turn the phone on, I have to do the same action. However, the same sound will only be played after about two seconds of holding down the power button. The delay is different. This means that if the phone is off and I hold the power button for one second it will turn on, but will not make the sound.
The reason for this seems pretty obvious to me, although from here on I’m just guessing: the difference in the delays is not a choice the designers of the phone made. It was forced by the fact that the moment the phone is turned on, it is still not ready to produce sounds. The operating system needs to boot and sound drivers have to be loaded.
So now a design decision: let’s say the phone is off and the user holds the power button for one second only. The phone is now on. Should it make the sound to alert the user that it is working or not? I would say that it should. However, the designers of the T630 decided that it shouldn’t. You can only hear it if you hold down the power button long enough.
The last thing I think about is how much code is there in the phone’s software to make sure the sound can me made as soon as possible, to give the user the best experience. The answer to that I’ll probably never know.