My understanding is that Google currently does not let you port your number to them, you set up a separate number through which calls can be funneled to and from your other phones (mobile, home, work, etc.).
I believe that one aspect of this is that since the Google number is not "in network" or part of your Family plan you have to use airtime minutes for call that might otherwise be free. This applies for both people calling you via your Google number and you calling out though your Google number or to check voice mail. If you have 'Friends and Family' or something like that where you can include the Google number among those listed then that can address this.
If you call someone direct rather than through the Google Voice app then that may also address the "in network" aspect but then you have the scenario of effectively having two mobile numbers that others may have to address, your 'direct' number and your Google number.
Since I have been giving my cell number to clients, business associates and friends for some years it was not real practical for me to implement a new number. In addition, we typically have a large number of minutes each month that are currently in-network or within plan, with Google Voice we might have to bump up to a higher airtime Family Plan to cover those. As a result, I didn't see Google Voice as a good option for me and chose to pay for Visual Voicemail. So far have been quite happy with it, but your situation may differ.