An option in the Sounds section of the iOS settings makes it possible to change the default ring tone for all calls you receive on your iPhone. Additionally, you can assign a different ring tone to specific contacts by editing individual contact entries. This works in all versions of iOS, starting from version 4 up to and including 8.1, the most recent version, at the time of publication.
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Changing the Default Ring Tone
Open the Settings app and tap "Sounds." You may have to scroll down for this option to become visible.
Tap the "Ringtone" option, found in the Sounds and Vibration Patterns section. This displays a list of all ring tones loaded on your iPhone.
Tap a ring tone in the list to select it as the new default. Selecting it also plays the ring tone, so if you don't like it, simply tap another one. Once you've found a ring tone you like, tap "Sounds" to return to the previous screen or simply press the "Home" button to exit the iPhone settings.
Setting a Ring Tone for a Contact
Open the list of contacts and tap a contact to display its information card.
Tap a ring tone in the list to play and select it. Once you've found a ring tone you like, tap "Done" to return to the previous screen.
Tap "Done" again to save your changes and return to the contact information screen.