When you sign up for Google Voice, or add a phone to your account, Google sends you a text message with a verification code. You enter this code to activate Voice on your phone.
Don't share your verification code with anyone
To prevent unwanted access to your account, it's important you don't reply to any requests for your verification code. Google employees will never ask for this code.
Remember that good security practices are critical to the integrity of your Google Account. Learn more at Keeping your account secure.