VoIP-to-PSTN termination providers and SIP vendors will be watching their inboxes for a lawyer’s letter from BT, which has kicked off a taxing licensing program levying a fee on the industry, based on a list of 99 patents.

via BT unleashes SIP licensing troll army • The Register.

A useful comment from slashdot.

The IETF MMUSIC (Multiparty Multimedia Session Control) Working Group started working on Session Protocols [ietf.org] in 1993.

Initial Internet drafts for a Session Invitation Protocol and a Simple Conference Invitation Protocol were prepared in 1996, and merged to a single first draft of SIP by December 1996 (slide 10 [columbia.edu]), with further drafts (2-12) leading up to the publication of RFC 2543 in March of 1999 (slides 11-13, ibid.).

I don’t see anything that says BT had a hand in anything to do with SIP up to 1996. More than half the patents BT claims (Exhibit C [btplc.com]) were filed after RFC 2543 was published.

I hope this information is a useful starting point for some SIP vendor.