[Serusers] "3xx Redirection" - How Should This Work?

Hi all. I'm trying to get my call working working and I have a question. Suppose a SIP subscriber has voicemail and call forwarding is enabled. When an incoming call is destined for this SIP user, SER will reply with a 302 redirect message. Next the caller's phone should construct a brand new INVITE to the redirected destination. My question is this: If the forwarded destination is busy, then the call should be sent to the voicemail of the SIP subscriber that was originally called. This is the way the RBOCs call forwarding works. If I'm using sl_send_reply("302", "Temporarily Moved") back to the caller, how can I make the voicemail piece work? Regards, Paul