Just for posterity, Alex is kind enough to help me out off list:
(05:57:00 PM) Alexandr Dubovikov: the method field must contain "405"
(05:57:08 PM) joegen.baclor: oh
(05:57:21 PM) joegen.baclor: so its not the CSeq method
(05:57:22 PM) Alexandr Dubovikov: and reply_reason must have "Method Not Allowed"
(05:57:27 PM) Alexandr Dubovikov: no
(05:57:33 PM) joegen.baclor: oh... ok. bummer
(05:57:35 PM) Alexandr Dubovikov: method is universal field
mystery solved