Some phone systems require the recording to be on a mp3 or wav file, and some you just have to record the greeting right into your phone. This last way is likely one you have tried already. Use the link here to find a FREE voicemail greetings for business. These are scripts you can modify for your use.
Consult your network manual for detailed information on changing your voicemail message.

A word of warning: These greetings will not do you any favors if you're in the midst of a job hunt or work in a conservative industry. Always remember your target personas. If there's a chance they won't appreciate your sense of humor, opt for a straightforward greeting instead.
Using your cell phone to record a voicemail message isn't always easy. Between remembering what to say and getting the right timing, it's tricky. But your phone does have a playback option, and it's easy to listen to and review your message. For some people, though, cutting off the end of a message is a funny way to trick people and catch them off guard. In most situations, callers won't know it's intentional and will think you're far less than tech savvy. Definitely not an impression I'd want to make.