After a lot of hunting around for a 3GP command line converter, finally tracked down a windows build of the mighty ffmpeg ( normally built for linux ). The trick with ffmpeg is to get a version that has the specifc audio convertor for 3GP ( the file format used by most mobile phones )
Because I am nice, I've included a ZIP file containing all the things you will need.
The following is a simple command line to encode near any type of video to 3GP:
( Replace source,temp and dest with relevent filenames ... dest MUST have a 3GP extension )
mencoder source -ovc lavc -oac lavc -lavcopts vcodec=mpeg4 -vop expand=176:144,scale=176:-2 -o temp -ofps 12
ffmpeg -i temp -s qcif -r 12 -ac 1 -ar 8000 -b 30 -ab 12 -t 60 dest
This calls 2 programs, mencode to encode the source to MPEG4 .. why ... because ffmpeg does not handle some formats or errors very well - mencode just cleans it up.
The ffmpeg encodes the new file to 3GP ... done!!!