Windows 7 media player 12 does not play .3gp video files

You will need a codec to play the video file. A codec is software that is used to compress or decompress a digital media file, such as a song or video. Windows Media Player and other programs use codec to play and create digital media files.
You may refer to the below link for further guidance on the issue-

http://windows.microsoft.com/en-US/windows7/Codecs-frequently-asked-questions

or

http://download.cnet.com/Windows-Essentials-Media-Codec-Pack/3000-13632_4-10662709.html

You may use the below provided link and check if you get the required codec to download as per the file type you are trying to play -
http://www.microsoft.com/downloads/en/default.aspx

You may also use your favorite search engine to download and install a codec as per the file type you need to use.

NOTE - Using Third Party Software, including hardware drivers can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the use of Third Party Software can be solved. Using Third Party Software is at your own risk.

Microsoft claims that Windows 7 with media player 12 will play .3gp video file formats. I have found this to be impossible, even after consuming hours of my valuable time seaching websites for answers, including microsoft's own help desk.

It is a disgrace that I have spent hundreds of pounds purchasing a brand new computer with the very latest software, yet it will not play basic video files created on a simple mobile phone!

Can anyone advise how to make Media Player 12 play .3gp video files?

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No guarantees but it may be worth a try to download
and install the following freeware codec pack:

(FWIW...it's always a good idea to create a System
Restore point before installing software or updates)

*Proceed At Your Own Risk*

K-Lite Codec Pack (Full)
http://www.filehippo.com/download_klite_codec_pack/

Good luck...