exten => _91NXXNXXXXXX,1,GotoIf($[${SIPTRUNK1} < 2]?SIPTRUNK1,s,1:101)
 exten => _91NXXNXXXXXX,2,Hangup
 exten => _91NXXNXXXXXX,101,GotoIf($[${IAXTRUNK1} < 5]?IAXTRUNK1,s,1:202)
 exten => _91NXXNXXXXXX,102,Hangup
 exten => _91NXXNXXXXXX,202,Congestion

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