Can Asterisk Force SIP codec renegotiations

Scenario:

  • SIP Phone A supports G711u and G729 default is set to G711u
  • SIP Phone B supports G711u and G729 default is set to G729
  • SIP Phone A Calls extension SIP Phone B on the same asterisk box.
  • Asterisk starts transcoding since A is using G711u and B is defaulting to G729.
  • Both phones are set to use G711u and G729 in Asterisk Extension config.

Is there a way to force asterisk to renegotiate the codecs between the 2 callers? I do not want asterisk to be the middleman.
Thanks.