We've seen a few situations in which Public Mobile has mistakenly ported in the alternate phone number by mistake due to some strange website or port processing error.
The alternate number is only supposed to be a contact number for Public Mobile to call should there be a porting problem. Public doesn't actually call the customer after a failed port (at least not usually), so my suggestion is for Public Mobile to just remove it from the port in number request form. It's doing more harm than good.