Condition outbound tunk busy next trunk

I have a question on my pbx. We have 4 analogue lines and 3 SIP accounts available. Sometimes when using the analogue lines for local calls all trunks are busy and we want to put a condition that if the outbound trunk is busy it will use one of the SIP accounts. How can i program this condition? I know it is a command but i searched all over the net and i cand find it. Basically it is the following:

MAKE CALL --> OUTBOUND TRUNK MARKS BUSY --> USE NEXT TRUNK

Anyone knows the command??

Greetings,

Richard

Outbound routes allow you to prioritize trunks used, just add them there.

Thank you very much… very easy!