While executing a consultative transfer, sipXtapi sends a REFER to one leg with a Refer-To URI that directs that leg UA to send an INVITE-with-Replaces to the other leg's UA. If the second leg's UA does not support Replace, it behaves in ways that can confuse the user. It would be more effective to have the second leg's UA reject the INVITE with a 420 (Extension Not Supported) error, as that error would be reflected back to the sipXtapi (in a NOTIFY), and the sipXtapi or its application would know positively that the operation had failed, and the reason it had failed.

To accomplish this, sipXtapi should include "?Requires=replaces" in the Refer-To URI in a consultative transfer.

Beware that we need to fix sipXtapi to tell its SipUserAgent that it supports "replaces" *before* we make this change. See XCL-100.