FFmpeg can "read" and/or "extract" subtitles from embedded subtitle tracks.

For instance, if you run ffmpeg -i <my_file> and you see something like:

    Stream #0:2: Subtitle: ssa (default)

or

    Stream #0:2[0x909](eng): Subtitle: dvb_teletext ([6][0][0][0] / 0x0006), 492x250

You can extract or convert those subtitles. To convert to srt from dvb_teletext.

ffmpeg -txt_format text -i input_file out.srt

Your FFmpeg needs to be configured with --enable-libzvbi for this to work, and results in something like this:

  Stream #0:17 -> #3:0 (dvb_teletext (libzvbi_teletextdec) -> subrip (srt))

vbi teletext is a format specification that allows for including text (characters) and a few basic images as metadata between video frames of a broadcast.

libzvbi_teletextdec can decode that, and output in two formats, either "image" output or "text" output. Default is image. Text output must be selected in order for it to be converted to other text formats, like, in this instance, srt. See ​here for more options.

You can also convert to .ssa format, etc. Here is how to list all the subtitle options available:

ffmpeg -codecs | grep subtitle
ffmpeg version 3.1.3 Copyright (c) 2000-2016 the FFmpeg developers
  built with Apple LLVM version 7.3.0 (clang-703.0.31)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/3.1.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-ffplay --disable-lzma --enable-vda
  libavutil      55. 28.100 / 55. 28.100
  libavcodec     57. 48.101 / 57. 48.101
  libavformat    57. 41.100 / 57. 41.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 47.100 /  6. 47.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
  libpostproc    54.  0.100 / 54.  0.100
 DES... ass                  ASS (Advanced SSA) subtitle (decoders: ssa ass ) (encoders: ssa ass )
 DES... dvb_subtitle         DVB subtitles (decoders: dvbsub ) (encoders: dvbsub )
 DES... dvd_subtitle         DVD subtitles (decoders: dvdsub ) (encoders: dvdsub )
 D.S... hdmv_pgs_subtitle    HDMV Presentation Graphic Stream subtitles (decoders: pgssub )
 ..S... hdmv_text_subtitle   HDMV Text subtitle
 D.S... jacosub              JACOsub subtitle
 D.S... microdvd             MicroDVD subtitle
 D.S... mpl2                 MPL2 subtitle
 D.S... pjs                  PJS (Phoenix Japanimation Society) subtitle
 D.S... realtext             RealText subtitle
 D.S... sami                 SAMI subtitle
 ..S... srt                  SubRip subtitle with embedded timing
 ..S... ssa                  SSA (SubStation Alpha) subtitle
 D.S... stl                  Spruce subtitle format
 DES... subrip               SubRip subtitle (decoders: srt subrip ) (encoders: srt subrip )
 D.S... subviewer            SubViewer subtitle
 D.S... subviewer1           SubViewer v1 subtitle
 D.S... vplayer              VPlayer subtitle
 DES... webvtt               WebVTT subtitle

zvbi is a decoder (only) so you can see it like

$ ffmpeg -decoders | grep zvb
ffmpeg version 4.1.3-0ubuntu1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8 (Ubuntu 8.3.0-6ubuntu1)
 S..... libzvbi_teletextdec  Libzvbi DVB teletext decoder (codec dvb_teletext)

The ones with capital E you can convert into (subrip, ass, webvtt), ex:

ffmpeg -i my_file.mkv -f webvtt outfile

or implied by file extension:

ffmpeg -i my_file.mkv outfile.vtt

or one subtitle type file to another:

ffmpeg -i my_file.srt outfile.vtt

If there are multiple subtitle tracks in your video file, then by default it will select only the first. You can use the [Map] command to select which ones you want.