It's got nothing to do with your phone number or telco, it's your phone's MAC address that is being recognised.
Either Stacey has a phone with the same MAC address or Stacey was the original purchaser of your phone and returned it to the store, whereupon the store did a factory reset and re-boxed it to be sold as new.
MAC addresses are supposed to be unique to each handset but there have been cases of phones that have the same MAC address ex factory.