asterisk users - May 2013 - ChanIsAvail function is breaking the round robin strategy

Hello everybody,

i have two gsm line (extra channels) and i'd like to schedule the
outgoing calls with a round-robin strategy.
If all the gsm lines are busy, the call must be sent to the pri lines
with a linear strategy.

here is the dialplan:

exten => gsm,ChanIsAvail(EXTRA/r2&DAHDI/g1)
 same => n,GotoIf($["${AVAILORIGCHAN}" = ""]?unavail,1)
 same => n,Dial(${AVAILORIGCHAN}/${CALLEDNUMBER})

The problem is that the ChanIsAvail function is breaking the round
robin strategy.
When this function is executed i can see in the console this message:

Hungup 'EXTRA/3-1'

and when the Dial function is executed only the EXTRA/1 channel is
selected, never the EXTRA/3 one.

that happen because ChanIsAvail affect the round-robin selection
marking a channel as just used when it has only been checked.

Anyone can see a solution to that problem?

Thank you very much, im using asterisk 1.6.2 and dahdi 2.2.1 on a
debian squeeze station.