asterisk users - Jul 2004 - Any way to change ring back behavior for call park?

Hi,
I have call center with 12 Polycom IP 600 SIP phones connected to *. 
When an incomming call comes in over the PSTN, 7 of those phones ring at 
the same time, and any of those 7 agents can pick up the new call. This 
works great. The problem is that if one of the agents parks a call and 
nobody picks up that parked call before the timeout (2 minutes),  the 
parked call rings back all 7 extensions, just like a new incomming call. 
The agents want a way to identify a parked call ringing back. I have 
searched the archives and the wiki with no luck. What I would like to do 
would be to be able to set "ALERT_INFO" in park ring backs, so that I 
can force a different ring tone when the park times out. If that is not 
possible, I would like the parked call to only ring back to the 
extension that parked the call.
If anyone has any good ideas, that would be great.

Thanks,

-Tor Roberts