Most of the mobile phones with video cameras produce movies in 3gp format. If you want to convert them to avi format you can follow the following steps.
a) Copy the movie from your mobile to your PC . Let the name of the movie be mymovie.3gp
b) - Install mencoder. It is part of the mplayer package and available in the mutiverse repository on ubuntu
$ sudo apt-get install mencoder
c) Covert the to avi with following command.
$ mencoder mymovie.3gp -ovc lavc -lavcopts vcodec=msmpeg4v2 -oac mp3lame -lameopts vbr=3 -o mymovie.avi''