SIP and IAX Trunk combination


I need to get the following working:
I have 1 Provider that provides me a SIP trunk and 1 with an IAX trunk.

I setup both trunks in freepbx and both are online and registered.

I also setup outbound routes for both. Now I have to setup extensions that work with both trunks, depending on the dialed number.
I setup SIP Extensions. Thy work well with the SIP trunk, but they don’t with the IAX trunk. When dialing a number that is routed over the IAX trunk I allways get a busy signal. The same thing when dialing in to the number related to the IAX trunk.

What do I have to do to get the possibility to use both trunks depending on the dialed number?