Google Voice question..???..???

I just got Google Voice working and i love it. The transcriptions arent very accurate but they are funny and fun to see. I was wondering though since it forwards the call to your google number to use the voicemail does it cost extra or use extra minutes? Ive seen that some people put the number in their friends and family numbers but i was wondering is that necessary? What will happen if i dont? How will my minutes/bill be charged?