Security experts at Symantec are warning about a new password recovery scam targeting Gmail users worldwide mainly for intelligence gathering. All an attacker needs to take over a webmail account is the victims email address and cell phone number. The attacker then sends the victim an SMS message saying Google has detected unusual activity on your account. Please respond with the code sent to your mobile device to stop unauthorized activity. The attacker can choose Get a verification code on my phone: [MOBILE PHONE NUMBER].”]
Source: http://securityaffairs.co/wordpress/37957/cyber-crime/password-recovery-scam.html