Information
Select 'Ask to record their voice to use as the notification' option for 'When someone joins by phone'.
Rationale:
This option enables Host and co-hosts to know that participants are joining/leaving while joining from phone.
Solution
Go into the Zoom Admin Dashboard on the zoom website. Account Management -> Account Settings -> Meeting -> In Meeting (Basic) -> Sound notification when someone joins or leaves -> When someone joins by phone -> Ask to record their voice to use as the notification, and ensure it is enabled.