Как настроить возврат из blind transfer при недоступности?

Верно. Attendant transfer можно использовать и как blind. В Asterisk 1.8.12.1 в features.conf есть опции:

;atxfernoanswertimeout = 15     ; Timeout for answer on attended transfer default is 15 seconds.
;atxferdropcall = no            ; If someone does an attended transfer, then hangs up before the transferred
                                ; caller is connected, then by default, the system will try to call back the
                                ; person that did the transfer.  If this is set to "yes", the callback will
                                ; not be attempted and the transfer will just fail.
                                ; For atxferdropcall=no to work properly, you also need to
                                ; define ATXFER_NULL_TECH in main/features.c.  The reason the
                                ; code is not enabled by default is spelled out in the comment
                                ; block near the top of main/features.c describing ATXFER_NULL_TECH.
;atxferloopdelay = 10           ; Number of seconds to sleep between retries (if atxferdropcall = no)
;atxfercallbackretries = 2      ; Number of times to attempt to send the call back to the transferer.
                                ; By default, this is 2.

См. здесь: Возврат звонка после безусловного / "слепого" перевода Callback after unattended / blind transfer http://linux.mixed-spb.ru/asterisk/callbackafterblind_transfer.php
если кратко, то:
1) можно сделать несколько Dial
2) можно настроить attended transfer как blind transfer
3) можно использовать TRANSFER _ CONTEXT

таким функционалом обладает Attended transfer