Change retry time out value for IAX

I have E1(PRI) connected to Asterisk and we are using IAX trunk service as well for out going call.
Here is extention.conf. Due to IAX trunk is down some time, we want to use ISDN when IAX is down.
Conf below works good but to identify IAX is down, it takes more than 6 secs.

exten => _0.,1,Dial(IAX2/USER:PASSWORD@iax.trunk-gateway.com/${EXTEN:1},120,T)
exten => _0.,n,GotoIf($["${DIALSTATUS}"=“CHANUNAVAIL”]?try_isdn)
exten => _0.,n(try_isdn),Dial(DAHDI/g1/${EXTEN:1},120,T)

Is there any way that you can change retry time of Dial IAX or is there any way to have multiple trunks with priority order?

Thank you for your help in advance.
Junic

Asterisk Ver 1.6.2.20

I mean caller need to wait more than 6 secs to place a call.