Using Google Voice, you can add your landline(s) and mobile phone(s) to your GV account. Let's say you've added one land line and one mobile phone to your GV account. When people call your GV number, it calls both of those numbers - so both your land line and mobile phone will ring. People can still call your LL and mobile directly. If they call your land line, only that phone rings. If they call your mobile, only your mobile will ring. In other words, your phones still work like they used to.
The problem I experience is that GV calls do not ring through. My phones ring once and then not again. Because my land line phone has caller ID announcement, I can hear the name of the person who is calling, but if I don't pick up the phone, it will not continue to ring.
I've configured GV to send me an email when calls are missed or voice mails are left. So far, it appears that only the former works as I have not received a single GV voice mail since FIOS service began.