Not an expert, but Verizon customer since Alltel
There should be an online activation, under her account, (which you may have to set up, will need an old bill for account info) that allows ‘bring your own device’ or equivalent. The ESN Hex number identifies that phone.
Or call Verizon. Or go to Best Buy geek desk. (Best Buy does Verizon, and others. Everyone working in cellphone department can do this.)
Point of reference , i have a Florida business landline number from 1978 that is still to this day my business number on an iPhone.