Recheck what you entered
Make sure the country code is right in the app. In some countries you have to drop the leading 0.
If the app wants to verify with a phone call rather than a text, it won't reach you. Switch back to SMS verification.
Keep waiting or switch
Codes usually land within a few minutes. If 2-3 minutes pass with nothing, tap resend in the app once.
Still nothing? Cancel the number in your dashboard and get one from a different country. The canceled number's price goes straight back to your balance.
Automatic refund
Leave it alone and the price still returns to your balance after 25 minutes. In My numbers it's marked "Refunded".
If one app keeps failing with numbers from several countries, open a ticket on the Support page with the app name and order number.