This has nothing to do with smartphones.
Last week I decided to create a Youtube account as their premium, ad-free service is now available in the UK. All was going well on my laptop until I hit the page demanding a phone number. Any number, smart or dumb.
Not having a burner-SIM to hand I just closed the tab.
Don't burners get their # recycled?
This seems like a pretty awful approach considering most of the sites I see demanding phone #s are doing so under the guise of improved security. It seems likely whoever controls that number will have some authority over the account.
All it would take is some stupid notification being sent to the phone number to inform whoever that is of something interesting being possible.
Combine that with the fact that burner phones are often utilized by criminals for variety of reasons...