[BCP76]    Best Current Practice 76,
              <https://www.rfc-editor.org/info/bcp76>.
              At the time of writing, this BCP comprises the following:

              Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and
              K. Summers, "Session Initiation Protocol (SIP) Public
              Switched Telephone Network (PSTN) Call Flows", BCP 76,
              RFC 3666, DOI 10.17487/RFC3666, December 2003,
              <https://www.rfc-editor.org/info/rfc3666>.