-I don't believe SipBroker supports IAX
-Some Asterisk setup suggestions
SIP Broker Wiki : Asterisk Configuration
-To make life easy you could add SipBroker as a trunk:
User: anonymous
Pass: <blank>
Proxy: sipbroker.com
The bonus is that any number pushed through SipBroker will undergo an ENUM lookup (all 4 servers I think) as well ... that said I am not sure how you'd set it up to fail the call over to another trunk if there is no ENUM record...