When you are using a telephone, you are speaking "on the phone." For short, "You are on the phone."
When you end the call, you "hang up" the phone.
After you hang up, you are no longer on the phone, so you are "off the phone."
If you want someone to stop using the phone, you want them to "get off the phone." You might say "please get off the phone, I'm expecting an important call."
Teenagers like to talk on the phone. Sometimes it has hard to get them off the phone. Sometimes it is a problem. The speakers asks if something "is the solution to getting kids off the phone."
P.S. Why do we say "hang up?" Try a web search for images of "candlestick telephone." A century ago, phone had an actual hook at the side. The hook held the earpiece. When you were done, you hung up the earpiece. The weight of the earpiece operated a switch which ended the call.