My understanding of this is a little different. I could be wrong, but this is what I understand:
For incoming calls: By default, GV displays the caller's phone number on your caller ID. With this setting, you are not saving any minutes adding your inbound GV number to your Friends and Family. Keep in mind that all your Google does when someone calls your GV number is forward the call to your phone (mobile in this case). So if I call you from 212-555-1234, and that number shows up on your caller ID, then it is like getting a direct call to your mobile number from 212-555-1234. All GV is doing is forwarding the number to your mobile. This will cause you to incur regular minutes to your calling plan (as if the caller called you directly without using your GV number)
If you have your caller ID always display your GV number when an inbound call comes in (this can be changed in your settings of your GV account), then having your GV number added to your Friends and Family plan will make all incoming calls free. You can then use the addition of Call Screening (in your settings of your GV account), you can have inbound callers announce themselves by saying their name so that you still get the benefit of knowing who is calling you even though your caller ID is showing your GV number.
For outbound calls: Like ShadowDude said, when Google updated GV (2 iterations ago) they added direct access numbers for each of your contacts. In the past, you had 1 static outbound number, that if added to your Friends and Family plan, would result in all of your outbound calls being free. Now that you have individual direct access numbers for every number you dial, you only have 3 options (that I can see).
1. Use an app like Clementine suggested (Google Voice Call Back)
2. Take the people you call most and add the direct access number that GV assigns to them and add that to your Friends and Family plan. Remember that you are limited to 10 numbers, so this is not ideal.
3. Uninstall the current version of GV on your phone and download the APK file for version 0.3.0. This is the last version that used a static outbound number. Keep in mind that you will be prompted every few days to update to the new Google Voice app from the Market, but as long as you have Automatic Updating turned off, you will be fine. It is a minor annoyance, but probably your best option if you are looking for your outbound calls to be free.
Here is an article about this from Phandroid where they link you to the 0.3.0 APK file at the end of the article:
Google Voice Update Brings Direct Access Numbers for Speedier Dialing, Eliminates Free Calling Trick | Android Phone Fans
Hope this helped