Can not stream g.729 using RTP

[Kevin Matthews]

Summary of the bug:

I'd like to send an RTP stream of pre-encoded G.729 audio data.

How to reproduce:

% ffmpeg -re -i encoded_audio.g729 -acodec copy -f rtp rtp://<address>:<port>
ffmpeg version 3.4.6-0ubuntu0.18.04.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 7 (Ubuntu 7.3.0-16ubuntu3)
  configuration: --prefix=/usr --extra-version=0ubuntu0.18.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-librsvg --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libavresample   3.  7.  0 /  3.  7.  0
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
[g729 @ 0x5624a3128900] Estimating duration from bitrate, this may be inaccurate
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, g729, from 'encoded_audio.g729':
  Duration: 00:00:09.84, start: 0.000000, bitrate: 8 kb/s
    Stream #0:0: Audio: g729, 8000 Hz, mono, s16
[rtp @ 0x5624a312c040] Unsupported codec g729
Could not write header for output file #0 (incorrect codec parameters ?): Operation not permitted
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
    Last message repeated 1 times

ffmpeg doesn't implement a G.729 encoder, but I thought that the copy codec would circumvent encoding/decoding and simply mux the audio.

There is a short discussion of this here:
​SuperUser.