How free public numbers work
A free SMS site publishes a handful of numbers and displays every message they receive on a public web page. Nothing is reserved for you: you and everyone else on the internet are watching the same inbox at the same time.
That is a real service with real uses — testing your own SMS integration, checking whether a form sends anything at all, or throwaway sign-ups you genuinely do not care about. Within those limits, paying would be a waste.
Why they fail verification so often
Two mechanisms defeat them. The first is privacy: the code is visible to strangers, so anyone watching that page can complete your sign-up or take over the account before you do. For anything that matters, that alone disqualifies them.
The second is blocking, and it is structural. A public number is used for thousands of registrations across every major platform, so platforms notice, and the entire number gets added to a blocklist. By the time a free number is listed on a popular site, the services you actually want to verify have usually already refused it. That is why free numbers appear to "stop working" — the number did not break, it got known.
What you are actually paying for
A paid activation buys two things a free number cannot provide: exclusivity for the duration — the number is yours alone, so no other customer receives its messages — and a fresh, less-burned number, which is the entire reason acceptance rates are higher.
You are also paying for stock. Keeping numbers available across many countries at once costs money continuously, and that is the real difference between a service that has a number when you need one and a page of numbers that all stopped working last month.
How to not overpay
Only pay for successes. On a charge-on-delivery model you reserve the price and are billed solely when a code actually arrives — a rejected or silent number costs nothing, so failed attempts are the provider's problem rather than yours. Prefer that over any service that bills at issue.
Do not automatically buy the most expensive country either. Match the country to the platform: an expensive number for a platform that happily accepts cheap ones is wasted money, while the cheapest number in the list is usually cheap because acceptance is poor.
A straight recommendation
Use a free public number for testing and for sign-ups whose loss would not bother you. Use a private paid number when the code has to arrive and stay private — a sign-up you want to succeed, or a service you would rather not hand your personal number to. Never use either for banking or payments, or as the recovery number on an account that matters.