If you are concerned about calls coming in to ex-employee numbers, then the idea of having multiple answer points, one per ex-employee, is a bad one.
You should instead have one answer point for all calls coming in to ex-employee numbers.
You could use the call-rerouting form or the DID form and send the call to a general office/reception number.
Or send it (via a nametag huntgroup if necessary) to a single NuPoint voicemail box where you can have a callflow that plays the messaging you want them to hear, giving them an opportunity to enter a different extension number to be put through to, or transferring the call to reception.