If you entered a free number and no code appeared, the cause is almost always one of five things. They are worth knowing in order, because the fixes are very different.
1. The number has stopped receiving
Public numbers are rented and rotated constantly, and one that worked last week may be off the network today. Every number here shows whether it is Live, Slow or Archive, based on when it actually last received a message. If a number is marked Archive, nothing you do will make a code arrive on it — pick one marked Live instead.
2. The service refuses shared or virtual numbers
Many large services detect that a number belongs to a bulk SMS provider and silently decline to send to it. You will usually see the request appear to succeed with no message ever arriving. WhatsApp, Telegram and some banks are strict about this; smaller sites and one-off sign-ups are far more permissive.
3. Someone already registered that number
These numbers are shared, so a popular one has often been used for the same service already. The service then treats it as an existing account rather than a new sign-up. Trying a different number — ideally one with fewer messages — usually resolves it.
4. The code arrived and scrolled away
A busy number can receive several messages a minute. If yours landed while other traffic was coming in, it may be further down the list. Use the search box on the number's page to search its full history for the sender name or part of the message.
5. You are waiting on the wrong country
Some services only send to numbers in the country the account is registered in, and will refuse a foreign number outright. Choose a number from the country the service expects.
How long should you actually wait?
Most codes arrive within a few seconds. If nothing has appeared after two minutes, it is not slow — it is one of the reasons above. Switch numbers rather than waiting longer.