Many apps will not let you in until you confirm a phone number. A temporary phone number solves this: it receives the code for you, and your own number stays private. Here is how to get one for free.
Open the home page and pick the country the app expects. Then choose the app you are signing up to. A temp number appears at once, and it is held for you for 15 minutes.
Copy the number and paste it into the sign-up form. Keep this page open. The app sends its SMS to the temp number, not to your phone.
The code shows on the page, usually within a minute. Type it into the app and you are verified.
Free temp numbers are shared, so use them for sign-ups and trials. For an account you want to keep, rent a private number that only you can read. You pay only when the SMS arrives.