ChanIsAvail

Dear sirs,
using ChanIsAail i can get the names of unused channels, for example:

exten => s,n,ChanIsAvail(SIP/LINE1&SIP/LINE2&SIP/LINE3&SIP/LINE4&SIP/LINE5&SIP/LINE6&SIP/LINE7&SIP/LINE8,as)

I need to select random line from available unused lines… Is it possible? And does anybody help me?