Something

globals]
EMERGENCY=0
EMERGENCY_TRUNK=Zap/17
; Change this for production use:
EMERGENCY_NUM=some_test_phone_number

[nineoneone]
exten => s,1,SetVar(SET_EMERG_FLAG=0)
exten => s,n(checkavail),ChanIsAvail(${EMERGENCY_TRUNK})
exten => s,n,SetGlobalVar(EMERGENCY=1)
exten => s,n,SetVar(SET_EMERG_FLAG=1)
exten => s,n(dial),Dial(${EMERGENCY_TRUNK}/${EMERGENCY_NUM})
exten => s,s+2(trunkbusy),GotoIf($[${EMERGENCY} = 1]?inprogress)
exten => s,n,SoftHangup(${EMERGENCY_TRUNK}-1)
exten => s,n,Wait(12)
exten => s,n,Goto(checkavail)
exten => s,s+2(inprogress),Congestion
exten => s,checkavail+101(notavail),Goto(trunkbusy)
exten => h,1,GotoIf($[${SET_EMERG_FLAG} = 1]?3)
exten => h,3,SetGlobalVar(EMERGENCY=0)


[your_main_context]
exten => 911,1,Goto(nineoneone,s,1)
...etc...


Dialing emergency number (111 for me) just repeating.....as follows....

  -- Executing Goto("SIP/grandstream2-b9d9", "outbound-emergency|s|1") in new stack
    -- Goto (outbound-emergency,s,1)
    -- Executing Set("SIP/grandstream2-b9d9", "SET_EMERG_FLAG=0") in new stack
    -- Executing ChanIsAvail("SIP/grandstream2-b9d9", "Zap/g2") in new stack
    -- Executing Goto("SIP/grandstream2-b9d9", "trunkbusy") in new stack
    -- Goto (outbound-emergency,s,7)
    -- Executing GotoIf("SIP/grandstream2-b9d9", "0?inprogress") in new stack
    -- Executing SoftHangup("SIP/grandstream2-b9d9", "Zap/g2-1") in new stack
    -- Executing Wait("SIP/grandstream2-b9d9", "8") in new stack
    -- Executing Goto("SIP/grandstream2-b9d9", "checkavail") in new stack
    -- Goto (outbound-emergency,s,2)
    -- Executing ChanIsAvail("SIP/grandstream2-b9d9", "Zap/g2") in new stack
    -- Executing Goto("SIP/grandstream2-b9d9", "trunkbusy") in new stack
    -- Goto (outbound-emergency,s,7)
    -- Executing GotoIf("SIP/grandstream2-b9d9", "0?inprogress") in new stack
    -- Executing SoftHangup("SIP/grandstream2-b9d9", "Zap/g2-1") in new stack
    -- Executing Wait("SIP/grandstream2-b9d9", "8") in new stack
  == Spawn extension (outbound-emergency, s, 9) exited non-zero on 'SIP/grandstream2-b9d9'
    -- Executing GotoIf("SIP/grandstream2-b9d9", "0?3") in new stack

etc etc....