[macro-next_avail_channel]
exten => s,1,ChanIsAvail(SIP/${SIP_OUT5}&SIP/${SIP_OUT6}&SIP/${SIP_OUT7}&SIP/${SIP_OUT8}&SIP/${SIP_OUT9}&SIP/${SIP_OUT10}&SIP/${SIP_OUT11}&SIP/${SIP_OUT12}&SIP/${SIP_OUT13}&SIP/${SIP_OUT14}&SIP/${SIP_OUT15}&SIP/${SIP_OUT16}&SIP/${SIP_OUT17}&SIP/${SIP_OUT18})
exten => s,2,Cut(nextChannel=AVAILCHAN,,1)                                                               

[default]
exten => _00.,1,Macro(next_avail_channel)
exten => _00.,2,Dial(SIP/${EXTEN:3}@${nextChannel:4},240)

nextChannel always contains the same value. what's wrong ?