Attendee calling in to Zoom Webinars doesn’t work properly at the moment, so they have disabled that function.
In the meantime, attendees may connect to Webinar audio through the Zoom desktop client, web client, or mobile app. Panelists are unaffected; this is only for attendees.
You can read more about this on Zoom’s status page.
They say they are hoping to turn on this capability again in the next few weeks. Sorry for the inconvenience!