Transfer error codes (mpbx-b2bua)

Returned by B2bua::transfer, the call-control way to transfer a live call (W3-TEL-01; REST: POST /api/v1/call-control/calls/{id}/transfer and .../park, W7-API-01, call-control.md). A REFER from a phone gets SIP statuses instead (table in Transfers and hold). Messages never contain numbers, names or SIP identifiers. Guide: Transfers and hold.

Code	Kind	Meaning	What to do
MPBX-XFER-CALL-NOT-FOUND	not_found	No live call with this id in the company (a call of another company is never found).	Check the call id; the call may have ended.
MPBX-XFER-STATE	conflict	The call is not answered yet, is ending, or another transfer (or a transfer to voicemail) is in progress.	Wait for the answer or for the other transfer to finish.
MPBX-XFER-FORBIDDEN	permission_denied	The transferring party is not a device of the company (a trunk party never transfers), it has hung up, or the call is an emergency call (never transferred).	Transfer from the device's side of the call; emergency calls cannot be transferred.
MPBX-XFER-TARGET	invalid_argument	The target is not a dialable number, or the company's dial plan refused it for the transferring party (calling permissions, toll-fraud gate, an emergency number, no route, a busy extension or a full company cap).	Check the number and the transferring user's calling profile.
MPBX-XFER-UNSUPPORTED	unavailable	A transfer to voicemail, but this engine runs no media app or no voicemail hook.	Wire mpbx-media and the voicemail service into the engine (follow-up FU-TEL01-1).