You can set the default ringtone for calls and Clock alarms and timers. You can also assign individual ringtones to contacts so you know who’s calling.
Set the default ringtone:
In Settings, choose Sounds > Ringtone, then choose a ringtone.
Assign a ringtone to a contact:
From Phone, tap Contacts and choose a contact. Then tap Ringtone and choose a ringtone.