Your ringtone file must be .m4r file extension because iPhone reads .m4r as ringtone file. You can find .m4r ringtone from many sources. Example: Zedge, there are tons of free iPhone ringtones for download.
- Look for a ringtone you like on Zedge and click Import to iTunes button to download which is in .m4r format.
- Open iTunes, click Edit and then Preferences.
- Tick Tones box and click OK.
- Back to iTunes main screen, the new Tones options will appear under Library. Drag and drop your .m4r ringtone into Tones folder.
- Plug in/connect your iPhone, select Tones tab and tick Sync Tones box. Click Apply to start syncing.
- When it is done, the new ringtone is already stored on your iPhone (Settings > Sounds > Ringtone).