Google Adsense will accept new applications only if you provide a valid phone number. New Adsense applications will not be accepted if they do not provide their phone number. Google then verifies the phone number automatically.Google sends a phone verification PIN number to your email address. Click a link provided in the email to open the Phone Verification Setup page, from which you'll be able to initiate the call or set a convenient time for our system to call you.
You'll need to key in a 6-digit personal identification number (PIN) using your touch-tone telephone. Google gives a limited number of PIN entry attempts.
This Phone verification PIN is different from the Address verification PIN that is mailed by post to each AdSense publisher when earnings have reached $50 on their account. The Address PIN is to verify publisher contact address and is sent by snail mail.
Source: Google AdSense Phone Verification
