asterisk users - Aug 2005 - sending dtmf tones to the caller (not the called)

for the particular configuration of software/hardware that connects to 
my asterisk pstn gateway I need to do something like the following :

[...]
exten => _X,3,Dial(CAPI/02xxx.b${EXTEN},60,M(senddtmf))
[...]

[macro-senddtmf]
exten => s,1,SendDTMF(*)

but the DTMF must be sended to the caller channel, and not the called :

SIP -> * -> ISDN

SIP calls some ISDN number, when ISDN picks the call, is SIP which needs 
to hear the dtmf (to trigger the local billing monitor counter)

I can't find a way to send the tones "in the opposite direction",
any idea ?