asterisk users - Apr 2009 - Sending Re-Invite with Dialplan application?

Hi,
  I have a requirement where an IVR application on asterisk has to play a audio
file in g729 and when a digit is pressed, the call should switch to another
codec (say ulaw). So, What can I do in the extensions.conf to trigger a
re-negotiation of codec?

I used 
exten => 55xx,n,Set(SIP_CODEC=ulaw)

but, I suppose this affects the next call and not the current one.

Please help ASAP

Thanks,
Sai



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