Lots of older phones don't support MP3 files. Some of the newer phones also don't support MP3 files. So, you can't just upload your MP3 file to your phone and guarantee that it will work. However, please feel free to try, and I wish you luck.
Phones that do not work can give you an error message such as Data Error 907, or they might say that you need an additional plugin in order to handle that particular MIME type. As far as I know, you won't be able to get any additional plugins for you phone.
If you do want to stick a MP3 file on your phone, I suggest that you make it smaller. Try to trim it to 30 seconds or less. Also, since phones don't have the best sound quality in the world, maybe decreasing the audio to mono and 96 k/s would be a good idea.
There is a good guide on fusicringtones that shows you step by step how to make MP3 ringers.
If you have a Sprint phone, you can try to convert the sound clip into QCP format and see if that works.
If you have an LG phone (like the PM-225 or 5350), you can just rename the file from an .mp3 to .qcp and try uploading that. It turns out that LG phones have a "block" of sorts. They do not let you download .mp3 ringers, but if you get an .mp3 song into the phone, they will let you play .mp3 files just fine. I suggest you convert your .mp3 into a .wav file, trim it to 25-30 seconds, and then reconvert it back to 32 or 48 k/s, 22,000 hz, mono. [Tip submitted by Brad729]