The phrasal verb "get off" (get) has more than one meaning. In your example, it means to cease or stop an activity. "Get off the phone, we have to leave." It can also mean to receive almost no consequences. "He was lucky to "get off" with no punishment." As a cautionary note, it has a slang meaning that is adult in nature. To "get off" during sensuous adult activity is to achieve physical pleasure. Italki won't let me explain it in more explicit (not vulgar) ways.

We use on/off (get on/off) for interactive communication devices, generally. Get on/off the computer/phone/radio. For the radio example, this would only be if you're speaking on the radio (same for the TV). If you're just listening to the radio, we wouldn't say to get on/off the radio--instead, you'd just be listening or, "stop listening to the radio.". Same with the TV (watching instead of listening).

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.

This is a rather difficult sentence, and not a particularly good one, but it can be analyzed. Many sentences with "get" are hard to interpret because "get" is such an imprecise verb. It sort of means whatever the speaker wants it to mean. For that reason, different people will say different things about what the sentence means and they will all be correct. My interpretation for this example is that "get on" and "get off" are used in the sense of addiction. For example, you could say "Billy has a bad habit. He got on some bad drugs but he is trying to get off of them". Cellphones, for many children, are like a bad drug to which they become addicted. Thus you could say "I am trying to get Billy off of his cellphone" to mean you are trying to break Billy's bad habit of overusing the phone.

*if I corrected