On iPhone, there is no way to select any ringtone but those which are available within WhatsApp. You can change the ringtone for when the app is closed via WhatsApp Settings > Notifications. Here you can choose different ringtones for messages, group messages and WhatsApp Calls.

With iOS 10, iOS 11 and iOS 12 and third party call integration, the ringtone used for WhatsApp voice calls is the ringtone set in the contact information within the phone's Contacts app.

To set a custom ringtone for WhatsApp voice calls on iOS 10, iOS 11 and iOS 12:

1. Go to your iPhone Contacts app.
2. Tap the contact you wish to select a custom ringtone for.
3. Tap Edit on the top right corner of the screen.
4. Choose a ringtone.
5. Restart your iPhone.

Note: Custom notification sounds for messages can be set in WhatsApp Settings > Notifications.

Important: Due to a bug in iOS 10, your notification sounds may not properly play until you restart your iPhone (press and hold the power and home buttons for 10 seconds).