Find the option to set call ringtone or notification ringtone. Select the MP3 ringtone you downloaded from the available options.Be sure to save the settings after choosing the new ringtone. Your phone is now personalized with a unique ringtone.Call your phone from another device to test the new ringtone. Check if it sounds as expected and adjust if necessary.Do not hesitate to change your ringtone regularly to avoid monotony. You can even create a rotation of ringtones for different occasions or moods.