I'm pretty sure you can set up your calls to your landline to forward to another number after disconnection. There is a cost and I don't think it's permanent. If you have a Verizon white pages I'm almost positive I read about this service in the beginning section.
I'm sure after a period of time Verizon or some other company would want to use your landline phone #.
I'm wondering just out of curiosity if someone could disconnect their landline and port the number over to a cell phone?
We can do what's called an 'incercept' - meaning calls sent to your old number get a message with a number to call you at.
It lasts about six months, or however long it takes for your number to be recycled back into the pool of available BTNs.