[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