[penagwin]

Plus how would we share this "pin"? Maybe we could append it to the phone number, like "+1 (616) 281 - 2123#32343" - Oh wait, it's already a number, what's the point of adding more onto it? It would literally turn into the same system that we have, as you have to share the pin with everyone anyway.

[Evidlo]

It's different because then not every "number" will result in a call being placed. Determining whether a number is valid requires making a call, which ties up robocallers.