If you are trying to sign up with Google AdSense and you cannot because of problems verifying yourself via the phone verification step, do not worry, Google is aware of it.
The other day a publisher posted a thread at Google Groups with the bug report. An AdSense advisor has come into explain they are aware of the issue and working on it.
Our payments specialists confirmed that we're currently experiencing intermittent outages with our phone verification system. They and I are sorry for any inconvenience this causes you.
Thank you to everyone who has notified us of this issue, and we assure you that we're working to fix the problem ASAP. If you've been unable to verify your phone number, please follow mariaestrel's example and contact us through the AdSense Help Center or your account. A specialist will respond as quickly as possible and work with you to resolve the issue.
Then AdSenseAdvisor made an appearance at WebmasterWorld about this problem, but most people don't understand what phone verification is.
In short, if you are an AdSense publisher already, you have nothing to worry about. This is just to verify new publishers, as part of the sign up process.
More on the phone verification process at Google AdSense help.
As part of our effort to protect the accounts of AdSense publishers and provide account-specific support, a valid phone number is required for publishers. We use automatic telephone number verification to ensure that your information is accurate and up-to-date.
Phone verification is completely automated - you'll only need to key in a 6-digit verification code using your touch-tone telephone.