Trunking Groups...help?

Hello everyone,

I just deployed my first asterisk server yesterday and its running great. I have a problem though…:

I have two VOIP providers. One uses my ZAP interface, and the other is SIP based.

For my outgoing calls, how can I make it so if someone tries to call out and the default outgoing line is in use, it switches to the secondary outgoing line.

My default outgoing line is the ZAP interface, so if someone tries to call out when that is in use I need it to drop to the SIP provdier for outgoing calls.

Thanks!
Bill