iPhone requires .m4r extension for ringtone. To create a customize ringtone, please do the following
1. Get your mp3 or any other sony file
2. Edit it to your liking. Please note that you have only 40s of ringtone
3. Use program such as Wavepad to edit is good
4. Convert the mp3 file to .m4a file.
5. Use program such as Swtich Sound Converter is good
6. Rename the .m4a to .m4r
7. Sync this .m4r file with iTunes to your iPhone