It is a VERIZON phone so I don't think it can be used on any other company right now. Congress passed a law last week that phones have to be "unlocked" if the owner asks for that, but I am not aware when that law becomes effective. So, for now, it has to be used with Verizon.
No. There are no bills attached. My daughter got an iphone and stopped using this one, so you will have to get another number assigned to it by Verizon. You cannot use the previous number, because it was transferred to the iphone. It has never been reported as lost. When she changed phones, we put it in a desk drawer.