People will understand what it means. When an individual “switches off” he stops paying attention; he stops listening.
*The lecture was so boring that many people switched off after five minutes. *Whenever I am with my aunt, I tend to switch off.
Your example suggests that the “subscriber” is not picking up the phone because he is not paying attention. But that is not the case here, is it? He is not picking up the phone because he has switched off the phone; as a result, he cannot hear the phone ring. It is not the subscriber that is switched off, but the phone. A better way of saying the same thing would be, “The subscriber that you are calling has switched off the phone”, or “The number that you are calling has been switched off.”