GV is also commonly used as a primary phone number router. Anyone calling that number is redirected to one or more phones you specify. You can program how the routing works.
Lastly, GV can be used as an alternate voice mail service. That's how I use it on my son's phone. I configured his phone to forward unanswered calls to Google Voice instead of phone voice mail. If he doesn't pick up the call is forwarded to GV, which answers after one ring (it doesn't, for example, create a loop by forwarding back to the mobile). [SEE UPDATE]
I added this GV number to our AT&T Family "A List", so (assuming AT&T actually allows a GV number!) there are no minutes associated with forwarding to this number (though forwarding will use up minutes on the plan, so it is charged like standard voice mail).
There are many ways to play with a GV setup, I'm sure I'll tweak this configuration further.
Update: There's always a catch. It occurred to me that, as far as GV "knows", the call is coming from a validated mobile phone that GV forwards too. The default GV behavior is that validated phones, when calling in, go direct to the voice mail menu! So all callers would go directly to voice mail control. I had to use the Google Voice advanced options to turn off the 'go to voice mail' feature and instead require an asterisk and PIN to get to the admin menu. When using GV in this mode it's probably safer and simpler to remove forwarding/routing phones.