There is a lot my kid is going to do under my nose, getting a smartphone doesn't seem too likely (esp. if he has alternatives like a desktop).
Not YOU, specifically. I’m speaking in general terms, and somewhat from experience.
When I was in high school we barely had smartphones. A friend of mine had a Windows Mobile 6 device, some kids had Blackberry devices. The iPhone had just come out. Devices were somewhat less personal back then so my experiences may not be practically replicable today, but back in the day I would lend my phone to my friends at the end of the school day if they had been grounded and their phone taken away.
Again, that was almost 20 years ago (dear god), but there’s something to be said for the unchecked determination of a teenager in search of a gadget.
I don’t necessarily disagree with your stance, nor am I criticizing you. For every watchful parental eye, there’s always a bit of sleight of hand. It’s the way of so many of us here, and likely something passed on to the kids of many of us here.
Besides, I am pro dumbphone and don't plan on reading my kids' texts so I don't think these scenarios are applicable.
In the US at least, it is trivial to get a phone plan without being 18 or having an adult sign for it. There is no credit check nor identity check.
Just walk into any Walmart and pick up any one of a dozen prepaid kits. These plans require you to bring your own devices, but you can get a used/unbranded smartphone for under $100 and a plan with data for under $10/month.
It has been this way for at least 10-15 years, if anything post-pay/credit line plans are quite an outdated concept, and mostly used by older generations who are overpaying. Plus T-Mobile specifically have had three data breaches leaking all of their customer's personal information (which they wouldn't even have with a prepaid plan).
I got one at the bodega on the corner for $20. The owner wanted $35, but when I told him I'd pay cash, he took the twenty.
It was in packaging from some carrier in Mexico, but wasn't SIM locked. No problems for the last year.