Some Do Not Disturb (DND) attributes include directing the call to a preassigned extension (like a secretary or assistant), busy signal, DND signal, or recorded message generated by the telephone switch.
Some switches allow the call to go through to the extension but restrict the response to a visual indication.
Some PBX systems allow the assignment of DND circumvention codes to supervisors.
In some cases, the ringer just does not ring and the called party is thus not alerted.
Depending on the infrastructure, the caller may end up on the called party's voice mailbox, sometimes after a certain delay.