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ProxyTopic; see IANA registry. See also other UriSchemes. (hmm...?)

note that callto: and mailto: are misnomers. URIs are noun phrases. You can do other things with phone numbers than call them; for example, you could have a web page of your history with a caller pop up in response to the phone ringing (e.g. using caller id. (c) 1998 Jamie Zawinski <jwz@jwz.org> )

Some registration data from an old index, using NotationThree, assuming TheUsualPrefixes:

"mailto" is uriReg:schemeName of [
  dc:description """Electronic mail address""";
  dc:description """Electronic mail address; The mailto URL scheme""";
  is uriReg:specifies of [
    = <http://www.ics.uci.edu/pub/ietf/uri/rfc2368.txt>;
    dc:title """The mailto URL scheme""";
    dc:creator [
      dca:agentName "P. Hoffman"],
      [dca:agentName "L. Masinter"],
      [dca:agentName "J. Zawinski"];
    inet:rfc "2368";
    dc:date "1998-07"; ];