the only thing it really means is that all 3 lines are verizon owned. the one they are saying isnt their’s might belong to a reseller - kinda like tracfone is to wireless.
either way, i might know of a grey area way to get the one that isnt being used.
basically, port the number into a cell phone company, prepaid would prolly be better, something like virgin mobile. fake all the info needed for the port, see if it works. it might take a week or two to go from landline to wireless. then if it works, port that back to vz landline. you can get a virgin mobile phone from walmart to use prolly pretty cheap, just double check that you can return it after you open the package. you wont get airtime returned to you, but thats a minimal cost.
//standard disclaimer. this is all hypothetical. i am not endorsing this method, nor encouraging it. just a hypothetical theory that it might work