How is a PHONE NUMBER Chosen??

If I have an existing phone number, either another cellular number or
a prior landline, how i

How is my new cellular phone number chosen? If I have an existing
phone number, is it reserved by my prior company and then "purchased"
or something by Verizon to make it my new number? Or is it placed in a
pool used by all the companies, such as an "aging number", and then
assigned?

Does the employee enter YOUR old number, and then put in a request to
"grab" it as soon as it's release by the old company?

What if you dropped your old service a few days or a few weeks ago?
There's a possibility that it may or may not be available? Let's say
your friend had a number you wanted. Could you somehow "reserve" his
number after he dropped his service and then get that number assigned
to you?

If you know of a number that is unused for whatever reason, can you
just specify that number for your new phone? Or does the system do
some sort of check to make sure it used to be owned by you?

IOW, what's the interface to whatever system Sprint folks use to
assign a cellular number look like? Is there really a limit of four
numbers selected that a customer must choose between?

Thanks!

Mark