What are the free ways to confirm a phone number, for example, through a telegram bot?

Question

API

Let's say there is a WEB service, a phone number is required to enter, what ways are there to check that the client's number is? SMS = paid and not suitable, only free methods.
Maybe it's possible through Telegram? For example, a person subscribed to a bot, sent him any number from 3 to five, the Back server read the line and saw the user number, looked into the database, if there is such a number, then wrote down a temporary authorization code for him, if everything is fine, sent the bot what was registered , the bot told the client that he can enter.
Or something else that would be free.

Answer 1

Reset call from the pool of numbers, the client's task is to enter the last digits of the number.

Answer 2

If the toad is strangling, you can give the client a number to which you need to send an SMS with a code. Naturally, the client pays for SMS.

Answer 3

And I connected a tariff with unlimited SMS within Russia for 50 rubles and wrote a simple application for android.
The application constantly polls my site via api and sends SMS when necessary, that is, the smartphone itself sends SMS. The application was installed on an old smartphone and it has been working there for a couple of years.
And if someone registers from a non-Russian number, then the user is invited to call my smartphone (where my application is located) and the numbers of all incoming calls are transferred to my website. The smartphone does not pick up the phone, so the call is so free for the caller. Of course, confirmation by call is not reliable, but in some cases it can be used.
PS Any programmer without experience will write such an application in 1-2 weeks. An experienced programmer will do it in a few days.