asterisk users - Feb 2008 - Asterisk mishandling user busy isdn releases

Hello All,
I have found that Asterisk does not play a busy tone when it receives a 
USER BUSY ISDN RELEASE messages following an ISDN CONNECT message.

Details:
We are connecting our Asterisk PBX to a Lucent telephone switch via ISDN 
PRIs.  The telephone switch provides an authorization code feature where 
  every call originated from asterisk is immediately answered by the 
telephone switch, and prompted for an authorization code, before 
completing the call.  The sip phone registered with Asterisk dials a 10 
digit number, hears the prompt, and dials a 4-digit code.  The telephone 
switch authenticates the code and attempts to complete the call.  In the 
event that the far end is available the telephone switch passes the 
ringing tones inband over the pri.  In the event that the far end is 
busy,  the telephone switch sends an ISDN RELEASE message with the 
cause: USERBUSY.  Asterisk receives the RELEASE message and sends a BYE 
message to the sip phone and a RELEASE COMPLETE back to the switch. This 
does NOT result in the user hearing a busy signal.  Instead, the user 
hears a click and dead air as if they were hung up on.

Questions:
Is there a way to configure this behavior in the dial plan?

If not does anyone think that adding the following logic to Asterisk 
would be a useful patch?

If a zap channel is bridged with a sip channel, and the zap channel 
receives a user busy RELEASE, play a busy tone inband on the sip channel.

Cheers,
-- 
Ken W. Leland  III
Engineering
k3leland at monmouth.com
Monmouth Telecom