exten => _91NXXNXXXXXX,1,GotoIf($[${SIPTRUNK1} < 2]?SIPTRUNK1,${EXTEN},1:102) 
 exten => _91NXXNXXXXXX,2,Hangup 
 exten => _91NXXNXXXXXX,102,GotoIf($[${IAXTRUNK1} < 5]?IAXTRUNK1,${EXTEN},1:203)
 exten => _91NXXNXXXXXX,103,Hangup
 exten => _91NXXNXXXXXX,203,Congestion

[SIPTRUNK1]
 exten => _X.,1,SetGlobalVar(SIPTRUNK1=$[${SIPTRUNK1} + 1])
 exten => _X.,2,Dial(SIP/${EXTEN:1}@SIPTRUNK1)
 exten => _X.,3,SetGlobalVar(SIPTRUNK1=$[${SIPTRUNK1} - 1])
 exten => _X.,4,Hangup