The 3rd Generation Partnership Project (3G/3GPP) is a collaboration agreement that was established in December 1998. The 3GPP and 3GPP2 specifications call out specific types of media that can exist in .3gp and .3g2 files.
Once you move a video from cell phone to your PC, the easiest way to play it is to use Apple's QuickTime Player.
In general, if a cell phone record a 3g2 video, it should conform to the 3G2 standard. However, a cell phone may not has the ability to play all variety of 3G2 videos due to limited codec support on the cell. Thus, it is easy to convert a cell phone 3G2 to a video format playable on a computer, but it is not so easy to convert a PC video to a 3G2 which is compatible with your particular brand of cell phone.