I don't know if any of you have tried to verify your phone number lately, but I wanted to let you know that we're currently experiencing intermittent outages with our phone verification system.
Our payments specialists would like to thank everyone for notifying us of this issue and assure you that we're working to fix the problem as quickly as possible. If you've been unable to verify your phone number, please contact us through the Help Center or your account and we'll be happy to work with you to resolve the issue.
Its for new members who earned 50$. They can initiate a phone call which will give them a code which need to be entered. If you search for "Phone Verification" in Google adsene help you can see the article on it. Looks its replacement of PIN (Or may be addition)