Free receive-SMS sites publish a list of phone numbers. You pick one, type it into a sign-up form, then refresh the site's page to see incoming texts. There is no account and no payment — typically the sites are funded by ads. This page covers the category as a whole rather than any single site.
The catch is structural: the inbox is public. Every message sent to that number is shown to every visitor, including your verification code, and often the name of the service it came from. Anyone who sees that code at the right moment may be able to use it, and anyone can later request a password reset to the same number. The numbers are also used by many people at once, so popular platforms frequently refuse them or rate-limit them, and codes may not arrive at all. Our guide to receiving SMS online safely goes into the specific risks.
PhoneBorn is a paid alternative built on the opposite principle: numbers are assigned to one customer at a time and are never a shared or public inbox. For ongoing use, a plan gives you a real mobile number you keep. For one code, an OTP number is single-use and refunded automatically if no code arrives. PhoneBorn is not affiliated with any free SMS site.