Per their announcement in the Zoom Help Center (click to visit original article), Zoom has temporarily disabled telephone dial-in for webinar attendees only:
Disable telephone options for Webinar attendees - Windows, macOS, Linux, iOS, Android [platforms]
We have temporarily disabled the telephone options for webinar attendees in Zoom version 5.2.3 and plan to re-enable them in the next few weeks. In the meantime, attendees may connect to Webinar audio through the Zoom desktop client, web client, or mobile app. Webinar hosts and panelists can still join by telephone, as well as through computer audio.
Until Zoom updates this feature (as noted above, promised "in the next few weeks"), attendees cannot call in to webinars via telephone. They can still use their computer audio as normal.
This change does not affect:
- Meeting-type sessions (BOF sessions)
- Private meeting sessions
- Virtual tradeshow sessions
- Hosts, co-hosts, speakers, panelists (anyone with hosting powers)