Some (not all!) channel variables can be used in "field" with or without the ${} construct.
- username Directory User who has originated this call
- dialplan Which kind of dialplan is in use, almost always "XML"
- caller_id_name
- caller_id_number
- callee_id_name
- callee_id_number
- network_addr address from which this call has been originated
- ani Automatic Number Identification (actual caller number)
- aniii Automatic Number Identification 2 (actual caller number version 2)
- rdnis Redirected Dialed Number Information Service, previous destination_number of a transferred call
- destination_number
- source what module was used for the call, e.g. for a SIP call, mod_sofia
- uuid Universal Unique Identifier that denotes this ...