Example:
Station 101 has Forward All Calls going to station 105 by way of dial code 41. Meanwhile, station 101 also has Busy/No Answer set to go to voice mail via dial code 43. When a call forwards to 105 and times out, it knows to go to 101's mailbox due to the BNA forwarding.
However, if you change 101's BNA forwarding to go to 105 via dial code 43, then it has no way of knowing how to get to 101's VM.
