Assigning a custom ringtone allows you to distinguish incoming calls for different phone numbers on your account.
Note that this feature is currently only available on the web. Each user can select a custom ringtone for their assigned phone number without sharing the settings with other users.
Within the web app, go to Settings / Phone Numbers
On the Phone Numbers, select the phone number that you want to set custom ringtones
Select a custom ringtone from the drop-down
Click Save