asterisk users - May 2005 - lookup for extensions on another SIP Proxy

I've got * registered with 50 SIP extensions. There are two another SIP 
proxies. I'd like to configure following:

1. Call from outside comes on *. * looks up for an extension
2. If no registered extension is on *, then request is forwarded to SIP 
proxy 1.
3. If client in not found on SIP Proxy 1, then * forwards request to SIP 
Proxy2
4. If client is not found SIP Proxy 2 congestion tone is generated.

What is the best way to do it?

I.N.