Unknown Reason key in CDRs

In Asterisk, a null return usually indicates that the system could not determine or retrieve a valid value for that field or variable. If the value being requested isn’t set, wasn’t passed to Asterisk, or is not defined in the dialplan or channel variables, Asterisk will report it as null. For example, if a caller ID or variable isn’t provided by the SIP endpoint. In this case we show "Unknown reason" as reason key in CDRs.