For the most part it just connects your home phone to the local cell towers.
Yes you connect your home phone directly to device. Multi handset phones work best with this type of setup but not required. You can also hook the device up to home phone wiring if it has been PHYSICALLY DISCONNECTED at the NID. See.http://voip.mi-telecom.org/ for general instructions on how to do this. I am sure this is not a Ting supported option.
It will need a number. You can port your existing landlines number over to it. If you don't have a land line I assume Ting can supply you with a number for it.
Has no effect on cell usage. Well other than it uses your Ting voice minutes.