Is this number free to use?
Yes. It is a public line; anyone can enter it and read the inbox on this page.
Why is my code not showing?
The sender may block shared numbers, or the line already has an account on that service. Refresh once more, then take another online line.
Can someone else see my SMS?
Yes — the inbox is public. Codes are partly masked after 10 minutes. For a number only you can read, take a private one.