Asterisk or FreePBX Configuration

Hi, I need someone who can configure asterisk. What i want is basically very simple. When someone calls me, he shall hear a Wait-loop (Something like "Please hold the line, the next available agent....."). While the caller hears that, a SIP Phone (or several phones) have to ring. If nobody picks up within 30 sec. the call has to be forwarded to another Number. Everything is handled by SIP. I already have a proprietary PBX but it doesnt support that feature (but it supports SIP for both incoming and outgoing connections.) So i plan to route all incoming calls from my existing PBX to Asterisk. Asterisk shall pick it up and play the Waitloop and while it plays it, asterisk shall call back another internal number from my existing pbx which eventually let my phone ring :-). I already have FreePBX installed (but not configured), but i can also install linux and stock asterisk without gui if you prefer that. You need to configure it, that i have 10 numbers (virtual SIP phones) which i can use in my existing PBX for the forwarting TO-Asterisk and 10 numbers for calling my PBX back. (since i also have 10 external numbers) The original CallerID must be preserved so that i can still see who is calling. I hope i explained everything correctly. I'll provide SSH and webaccess to the asterisk box and the web frontend of my existing pbx for the SIP Configuration.