3.14.5 Message Processing Events and Sequencing Rules

When the user agent receives a REFER request in the INVITE dialog in which it previously advertised support for safe call transfer, as described in section 3.14.4, the user agent MUST examine the Refer-To header field of the REFER request. If the Ms-Sensitivity header field is present in the headers parameter of the URI in the Refer-To header field, the user agent MUST extract the Ms-Sensitivity header field and its value and add it to the INVITE request that it generates as the result of processing the REFER request.