In different revisions of the Polycom phone firmware, we have seen different ways in which an attempted blind transfer can cause a lost call.

The behavior I would prefer to see is:

    * Immediately remove the call from the user interface (when the
      'Send' is pushed) such that nothing the user does with the phone
      will affect the call, but..

    * Keep the SIP dialog information until you get either:
    * a BYE from the transferee (to which you should just
      respond 200), or
    * a NOTIFY with a final response.
    * For any 2xx response (reported in a NOTIFY), you can just terminate the
      dialog (send a BYE - if it were me, I'd save
      myself some trouble and let the transfer target
      send it, but either way is ok as far as
      SIPxchange is concerned).
    * For any response >= 400 (reported in a NOTIFY), the phone should
      ideally ring the phone, and when the phone is
      answered, re-INVITE to get the media back (take
      the caller off hold) so that the call is not
      lost.
    * With SIPxchange at least, you won't get a 3xx
      report, since the SIPxchange forking proxy will
      act on the redirect rather than forwarding it
      back to the transferee. If you do, ignore it.

Note that there should be no change in anything on receiving
notification of any 1xx response, including 180 Ringing; the fact that
some phone is ringing does not ensure that the call will ever be
answered, and if your phone looses the dialog information at that point,
then the call may be unrecoverable.

This has been reported to Polycom - see External Issue Reference