Some Marine Corps brigs use a third-party contractor for phone service, and if you get the call records they come in a file format that's a bunch of numbers, followed by a period, followed by some more numbers, followed by the extension gsm. If you try to play them in Windows Media Player on a government computer, they won't work.

They can be opened in Audacity, a free audio editor which can also export them to a more convenient format (WAV, WMA, possibly MP3 if you have an encoder as well). http://sourceforge.net/projects/audacity/

Also, the first string of numbers (before the period) corresponds to an internal UNIX timestamp, which measures seconds since January 1, 1970. This can be converted into a readable date format by using this website: http://www.onlineconversion.com/unix_time.htm