Change ringtone

If you're using an Android mobile, you can customize your incoming call ringtone on the Onoff app:

From the home screen, open the left-hand pane via the ☰ side menu at top left,
Choose "Options" under your number,
Then "Number settings" and "Ringtone".

Unfortunately, it's not possible to change your ringtone on iOS mobiles at the moment, but we're working on it!