Thought I would post ? here.
Years ago with one of my business I got a toll free phone number ported into to my cell phone.
The Company I got it from changed hands 3X.

I never received a bill.
Long story short after talking with the company which now owns the 1st company they told me it would cost more to find where the toll free # was being switched from then it was worth. They told me I have a toll free # with unlimited calls @ NO CHARGE.

What do you think the value would be if I sold the cell phone # which the Toll free # was ported into. ???????

If it is a vanity number (spells something) then you can get a few hundred unless it is more of a corporate like 1 800-KLEENEX and then it can be a few thousand.

If it is a run of the mill 800 number then the challenge may be that the carrier may decide not to continue the free service when you attempt to transfer the number to a new entity. And I am not sure anyone would want you to have a list of all their inbound calls or control over the number once they invest in marketing it.

So it's difficult to put a price on it. But a good start would be to compare what RingCentral is currently offering...

I transferred the cell phone 3x between 3 different cell phone providers .
800 number automatically went with cell # or followed

Portability is not the question. It is ownership and generally they do not honor transferring the free deals to new account holders.

I work for a company that offers telecom service and has an extensive history in the telecom business. If you know the number, there are websites out there that can tell you the responsible organization (RESPORG) for a given number, and that information can be used to track it down and get it ported to another carrier or change the switched destination.

You don't necessarily need a bill or any other information. When customers port numbers to our service, they sign a letter of agency (LOA) and our porting group takes care of it with our carriers. There are a lot of companies out there who can handle the work for you. Like others said, if it's an actual 800, 877, or 888 number, it's worth a bit more, as there aren't any new numbers of that kind to be had. We've descended to 844's now!