Outgoing Missed

I’ve been running a few tools (FreePBX, Asternic, etc.) to get information on outgoing calls. They all have a column called “Missed” in the out going call section:
Agent Total Completed Missed
Tom 14 12 2
Beth 12 11 1
Bob 22 13 9

What is a “missed” outbound call? A number dialed that never gets picked up? A busy tone? Something else?

Those statistic aren’t being created by Asterisk. Asterisk is a tool kit.