Block Numbers on a Teen's Cell Phone

Is it an android phone? Then all you are looking for is an app that blocks spam calls. I've Truecaller on my phone and it usually works.
On my daughters phones we use a parental control software that just blocks all the calls from numbers out of her cell phone contacts (I can manage these too with the parental control). I strongly suggest you to instal a parental control, there are problems not only with the unknown calls but also with messages, mails, websites, dangerous apps, in addiction to the cell phone obsession almost every teen has, you can limit the time too with parental control.

https://www.wikihow.com/Block-a-Number-on-Sprint

google it.
call the sprint store near you and ask

The do not call list is a joke. I have been on it for years, renew myself every couple of years, yet still get telemarketing and political calls. I even unregistered myself from voting and the political calls continue. Most phones/carriers don't have a blocking feature, but you can download an app that blocks and reports numbers as spam, so people getting a call from a number they have not blocked yet shows up as "spam" or "most likely spam" on the caller ID if they have been reported as such by others. You still have the option of picking up though, as sometimes, the power company's bill collecting number shows up as spam, but if you're past due on paying your electric, perhaps you'd appreciate the call and want to pick it up and find out. Once the number is blocked, you won't receive calls or texts from that person. The app I use is TrueCaller, it works great. Might be something for you to look into, if your phone or carrier don't support a blocking feature.

You can go to the do not call list on your computer and register your daughter phone number.

The internet is very helpful with electronic questions. Google by asking one question. Give them the brand and phone model.

My gmail has a spam folder. I move the message to spam folder. I rarely do that.

I block phone numbers on spam.

I have an android and can block numbers by clicking on the symbols to the left of the phone number. This pulls up the call. Along the bottom you click on block number.

You can put your phone number on the do not call list.this will stop calls from advertisers. Google do not call list.

You can call the provider and they will tell you how to do this. They won't do.it for you.

Just call your carrier and ask.