To customize a contact's ringtone on iOS, open your address book via the Contacts app or from the Contacts tab in the Phone app and open your contact's record.
Then press the Edit button, scroll down to the contact card options and then, in the Ringtone line, press Default. The ringtone selection menu will appear, choose a different ringtone than the one used by default for the other contacts and validate your choice by pressing the OK button.
From now on, when this contact calls you, the ringtone will be different from the other contacts in your address book.
To customize a contact's ringtone on Android, open the Phone application, enter the contact's card and press the options button with three small dots.
Press Set ringtone, select a ringtone for the contact and confirm your choice by pressing the Save button.
Repeat the operation as many times as necessary for all the contacts for which you want to have a particular ringtone.