[Asterisk-Users] Multiple Outbound SIP Trunks

[Pikoro]

Here is my situation:

I have an office with around 10 users.  Inbound calls will come in via 4 
PSTN lines.  Outbound calls will be routed across  a maximum of 10 SIP 
trunks.

How can I set up a "group" of outbound trunks which will rotate use 
dependant on how many outbound calls need to be made.

There will be no discrimination or routes based on outbound calling, 
like a certain trunk for international calls, another for local calls, 
etc... Only a group of 10 SIP trunks to be rotated for all outbound calls.

For example:

Customer Support person 1 makes an outbound call on trunk1 (selected 
randomly by asterisk).  Tech support person 1 needs to make an outbound 
call but for some reason is getting routed to trunk1 instead of to the 
next available open SIP trunk.

Can anyone offer any suggestions, links, websites, or conf files that I 
could refer to in order to make all of this work.

Thanks in advance.