Cau pleaase  help me making it work,   i tell u the scenerio u pls make it, i'll be grateful

4 Sip Gws

calls coming from SIP orignation GWs  to asterisk, asterisk forwarding them to termination GWs on SIP

GW Ips are 
192.168.0.2
192.168.0.3
192.168.0.4
192.168.0.5

i am previously handling it in this way

exten => _1234#91X.,1,SetCDRUserField(abbas)
exten => _1234#91X.,2,SetCallerID,1234
exten => _1234#91X.,3,Dial(SIP/${EXTEN:5}@192.168.0.2)
exten => _1234#91X.,4,Dial(SIP/${EXTEN:5}@192.168.0.3)
exten => _1234#91X.,5,Dial(SIP/${EXTEN:5}@192.168.0.4)
exten => _1234#91X.,6,Dial(SIP/${EXTEN:5}@192.168.0.5)
exten => _1234#91X.,7,Wait,1
exten => _1234#91X.,8,Hangup

due to any reason if first GW dont accept call, it goes to next
but since i opdated asterisk on 1.2.1   it hangs many channels on GWs and gradually all channels go busy and thers is no calls.

so i want to use  chanunavail and busy  now,   if channel not available or busy  on GW one  go to next.