This will show you what formats your version of ffmpeg
can decode (D) and encode (E).
Look for a line that says
I don't have any experience with this format, but it looks like if your ffmpeg is like mine, you should be able to encode in 3gpp format. Change .3gp to .3gpp in your ffmpeg command to try this.
DE amr 3gpp amr file format
You might also want to try mp4, and wmv formats since the documentation says the phone can handle these formats, and they are video formats as well.
For example, perhaps try
ffmpeg -sameq -i filename.flv filename.wmv