The enumeration SL_CALL_ORIGIN specifies whether a call
originated in the public network, or was transferred within a PABX:
- SL_CALL_ORIGIN_UNKNOWN
- Call origin is unknown.
- SL_CALL_ORIGIN_INTERNAL
- Internal call.
- SL_CALL_ORIGIN_EXTERNAL
- External call.
- SL_CALL_ORIGIN_OPERATOR
- Call originated or transferred by the operator.