Can you share more details about the call flow and debug if possible? Are you planning to replace an incoming disconnect code or you want to send an specific disconnect code?
<action application="set" data="continue_on_fail=TEMPORARILY_UNAVAILABLE"/>
continue_on_fail "Controls what happens when the called party can not be reached (busy/offline). If "true" the dialplan continues to be processed. If "false" the dialplan will stop processing. Can contain the return messages that will continue on fail also."
<action application="set" data="failure_causes=SERVICE_UNAVAILABLE"/>
failure_causes Controls which failure causes will be considered as a failure to the bridge(s). This will change the values for which continue_on_fail will fail by default unless continue_on_fail is set to true.
Depending of your flow you can use:
<action application="hangup" data="SERVICE_UNAVAILABLE"/>