How to convert video for viewing on Sony Ericsson C702?
Devices are often picky, and specifications are usually too uninformative so it's always trial and error. For example, your link indicates that the phone supports MP4 playback, but that is simply a container than can utilize several video and audio formats.
MPEG-4 part 2 video and AAC-LC audio
(partially based on the working video details you provided)
ffmpeg -i input -vf scale=320:-1 -c:v mpeg4 -b:v 400k -c:a aac -strict experimental -b:a 32k -ar 16000 -ac 1 output.mp4

Some recommendations:

Try increasing the audio bitrate since -c:a aac is not very good
for low bitrates, and because your build does not support better AAC
encoders.

Consider removing -ar 16000 and -ac 1. It's best to try to preserve the original file audio rate and number of audio channels if supported by the phone.

Try replacing -b:v 400k with -q:v using a value of or between 2 - 5 (lower is higher quality) to set a constant quantization parameter instead of an arbitrary bitrate.


Results may vary.
H.264 video and AAC-LC audio
ffmpeg -i input -vf scale=320:-1 -c:v libx264 -profile:v baseline -crf 28 -c:a aac -strict experimental -b:a 128k output.mp4

I'm unsure if this will work on your phone, but it's worth trying. See the FFmpeg and x264 Encoding Guide for more info on encoding with x264.