From aa-asterisk.org.uk wiki
Cause Codes are returned by (amongst other things) an ISDN line for call progress status and reasons for a call being disconnected. This is useful for fault finding purposes when displayed on the Asterisk CLI. Asterisk uses cause codes internally to record call progress.
Common Cause Codes
- 1 - Unallocated (unassigned) number. On BT lines, plays the message "The number you have dialled has not been recognised. Please check and try again."
- 16 - Hangup (normal clearing). Used when a caller or callee hangs up the phone in a 'normal' way
- 17 - Engaged. Should result in the engaged tone being played.
- 31 - Hangup (normal unspecified). Hangup for a reason other than the phone was hung up. Typically used by the mobile phone networks to signal when someone presses the 'red button' on a caller to stop it ringing.
- 41 - Temporarily out of order