Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van Wijk, "User Requirements for the Session Initiation Protocol (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired Individuals", RFC 3351, DOI 10.17487/RFC3351, August 2002, <https://www.rfc-editor.org/info/rfc3351>.