Number porting is a feature in which your existing mobile number will become your Google Voice number. It allows you to take advantage of the full set of the Google Voice features like, one number to ring all of your phones, online voicemail, and more. Number porting also costs a one-time fee of $20, payable via Google Wallet. Note that you will still need carrier service (Sprint, T-Mobile, Cricket, etc.) to receive calls on your mobile phone.