I don't have a lot of experience with the 3GP container, but I wanted to have a stub page to point to what little I do have.
You can see the Samsung Gravity 2 page where I encoded video successfully for that device.
Most importantly, the specs I used:
- AMR audio codec
- 1 audio channel
- 8000 hz audio
- 12k audio bitrate
- MPEG-4 simple video (H.263)
There's probably more support for it, but this is limited to my phone. It's a bit of a pain putting stuff on there, so I'm gonna pass on more testing.