D:\>ffmpeg -i Input.mp3 -acodec opus -vbr constrained -application voip -ac 2 -ab 32k Output.opus
ffmpeg version N-59852-g785dc14 Copyright (c) 2000-2014 the FFmpeg developers
  built on Jan 14 2014 22:02:00 with gcc 4.8.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetyp
e --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --ena
ble-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-l
ibopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libsp
eex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aa
cenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavp
ack --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      52. 62.100 / 52. 62.100
  libavcodec     55. 48.101 / 55. 48.101
  libavformat    55. 23.103 / 55. 23.103
  libavdevice    55.  5.102 / 55.  5.102
  libavfilter     4.  1.100 /  4.  1.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
[mp3 @ 02b2a7e0] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from 'Input.mp3':
  Duration: 01:23:01.89, start: 0.000000, bitrate: 111 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16p, 112 kb/s
Output #0, opus, to 'Output.opus':
    Stream #0:0: Audio: opus (libopus), 48000 Hz, stereo, s16, 32 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (mp3 -> libopus)
Press [q] to stop, [?] for help
size=   19792kB time=01:22:33.11 bitrate=  32.7kbits/s
video:0kB audio:19419kB subtitle:0 global headers:0kB muxing overhead 1.919336%