asterisk users - May 2004 - Indication Busy to a ZAP ISDN channel

Hi,

I am stuck with my extensions.conf and would appreciate a small hint from the 
ISDN experts.

What is the correct way to indicate a busy condition to a calling ISDN zap 
channel (TE410P) when a local SIP ext. is busy?

I have

[pstn-in]
exten => *****591,1,Dial(SIP/${EXTEN},45,r)
exten => *****591,102,Busy


and get

     -- Executing Dial("Zap/1-1", "SIP/*****591|45|r") in
new stack
     -- Accepting call from '*****0420' to '*****591' on channel
1, span 1
     -- Called *****591
     -- Got SIP response 486 "busy" back from ***.***.***.222
     -- SIP/*****591-ffeb is busy
   == Everyone is busy at this time
     -- Executing Busy("Zap/1-1", "") in new stack

*CLI> show channels
       Channel  (Context    Extension    Pri )   State Appl.         Data
       Zap/1-1  (pstn-in    *****591     102 ) Ringing Busy          (Empty)


if the SIP extension is busy. The calling ISDN party hears ringtone then instead
of busy indication.

Asterisk seems to know the ext. is busy but doesn't do the correct
signalling in
the zap ISDN D-channel.

What is the correct way to do this, of course without answering the channel and 
thus producing costs to the caller?


Thanks and regards,

Jan Baumann