asterisk users - Sep 2006 - error 407 authenticate on INVITE

Hi

  I noticed that in the normal flow of invite message, there is a
message 407 in between.

UA1 ---------INVITE ----------> AST1
       <--------407--------------
       ----------ACK------------>
       ----------INVITE----------->
      <------Trying--------------

There is a problem if 407 exists between asterisks.

UA1                                   UA2
  |                                         |
AST1 ---------INVITE ----------> AST2
       <--------407--------------
       ----------ACK------------>
      (AST1 don't know how to response the message 407 and the call will
drop)

I want to remove such authentication between asterisk such that call
can flow between them.  Is it possible to do that?  How?  Thanks!