asterisk users - Feb 2008 - from address modification

HI all,
  How can I modify the from address in sip message?  Say, I will a sip
account 1234.  I want to change the from address in sip message of
this sip account to 4321.
From: "4321" <sip:r321 at 208.81.31.259>;tag=as5b42e6
ango