which scritp sets the status of the lead to NA?

Im having trouble finding out which scritp exacly sets the lead status to NA.. can anyone please point me out to the right place?
I know users can set it to N but i need to know which part of the dialer stes it to NA when noone picks up
I know users can set it to N but i need to know which part of the dialer stes it to NA when noone picks up