Although the iPhone comes with a library of ringtones for calls and texts, the device is capable of storing custom ringtones created on your computer or bought through the Apple Store. As of the time of publication, the iPhone is only compatible with M4R filetype ringtones. When you import an M4R, it appears in the ringtones library pane of your iTunes media library. Transferring ringtones to your iPhone works the same as manually managing the music and videos on your device.
Connect your iPhone to your computer using the device’s USB connector cable, and launch iTunes if the application doesn’t automatically start.
Click “Ringtones” in the left side bar.
Drag the ringtone that you want to transfer to your iPhone from the "Ringtones" window pane to the left side bar and drop the item in the iPhone icon. The ringtone transfers from iTunes to your iPhone.