Been playing around with this for a while and found several examples online none of which worked for me in first try, got it working with the following command.
ffmpeg -i [input.avi]-acodec libamr_nb -s 176x144 -ar 8000 -b 120000 -vcodec h263 -ab 10.2k -ac 1 [output.3gp]
Enjoy,
