/usr/local/bin/ffmpeg -i myfile.avi -acodec amr_nb -ar 8000 -ac 1 -ab 32 -vcodec h263 -s qcif -r 15 out.3gp
FFmpeg version SVN-r8487, Copyright (c) 2000-2007 Fabrice Bellard, et al.
  configuration: --enable-amr_nb --enable-amr_wb --enable-liba52 --enable-liba52bin --enable-gpl
  libavutil version: 49.4.0
  libavcodec version: 51.40.0
  libavformat version: 51.11.0
  built on Mar 23 2007 23:13:41, gcc: 4.1.2 20060928 (prerelease) (Ubuntu 4.1.1-13ubuntu5)

Seems stream 0 codec frame rate differs from container frame rate: 23.98 (65534/2733) -> 23.98 (2997/125)
Input #0, avi, from 'myfile.avi':
  Duration: 00:24:23.1, start: 0.000000, bitrate: 1540 kb/s
  Stream #0.0: Video: mpeg4, yuv420p, 640x480, 23.98 fps(r)
  Stream #0.1: Audio: 0x2000, 48000 Hz, stereo, 192 kb/s
File 'out.3gp' already exists. Overwrite ? [y/N] y
Output #0, 3gp, to 'out.3gp':
  Stream #0.0: Video: h263, yuv420p, 176x144, q=2-31, 200 kb/s, 15.00 fps(c)
  Stream #0.1: Audio: amr_nb, 8000 Hz, mono, 0 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Unsupported codec (id=86020) for input stream #0.1