If you'd like to sign up for a Gmail address, you need to have a mobile phone that has text-messaging capabilities.
If you don't have a phone, you may want to ask a friend if you can use his or her number to receive a code.

One of the reasons we're offering this new way to sign up for Gmail is to help protect our users and combat abuse. Spam and abuse protection are two things we take very seriously, and our users have been very happy with the small amount of spam they've received in Gmail. We take many measures to ensure that spammers have a difficult time sending their spam messages, getting these messages delivered, or even obtaining a Gmail address (spammers will often use many different addresses to send spam). Sending invitation codes to mobile phones is one way to address this, as the number of addresses

Refer this url for details . SMS account verification - Gmail Help