Neither of these is correct. Do you mean that the phone is disconnected? If so, we would say "that subscriber (or number) is no longer connected". or "that number has been disconnected". Another way to say it is "that number is no longer available". The expression 'power-off' is used for things like airplanes and large machinery - they 'power-off' when they shut down. The opposite is to 'power up'. This expression is more likely used with something big, or something which takes a while to reach full power once it is turned on (like a computer). Otherwise we would just say 'turn on' or 'turn off' (as for example, an electric light. Does that help?