Hi,
With the help of NCH Technical Support, we've managed to make FTP working over 3g. 
We've simply had to uninstall / reinstall Pocket Dictate from iPhone and then problem solved.
Thanks a lot to NCH Technical Team for their help.
Best Regards.
EDIT : After a few more tests, it appears that sending dictations via FTP over 3g works 10% of the time, so it's not really usable.
'Hope NCH will release an updated version soon.