The From field of the response MUST equal the From header field of the request. The Call-ID header of the response MUST equal the Call-ID header field of the request. The CSeq header field of the response MUST equal to the CSeq field of the request. The Via header field values in the response MUST equal the Via header field values in the request and MUST contain same ordering .
If the request contained a To tag in the request, the To header field in the response MUST equal to that of the request. However, if it did not contain a tag, the URI in the to field must be equal to that of request and in addition, UAS must add tag to the To header field in the response. The exception to this is 100 trying, in which tag necessarily not present. The same tag MUST be used for all the responses to that request, both final and provisional.
references:
https://www.ietf.org/rfc/rfc3261.txt SECTION 8.2.6.2
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