imported post
They may be able to 'play dumb' in responding to a request about a particular call, and can later claim that since no crime was committed they didn't keep track of what data belongs to that call.
A nice way around this is to request access to ALL records for a particular period of time. Be sure you are requesting "ACCESS" and not "COPIES", or they can stick you with a large bill for the cost of making all those copies - most which are unrelated.
For example, you could request "all audio recordings of incoming calls to the Sunnyvale 911 emergency line between 0900 and 0920 on 09/11/2009." Chances are you'll only have to listen to a few calls before finding the one you want.
The same can be done for non-emergency lines (though not all PDs record all inbound lines), radio comms, electronic communications, etc.
Of course, they may tell you to pound sand anyhow. Your only further recourse is to sue.