Yes, there is a good technical solution for audio call-in by attendees, but it requires some equipment. Look up 'mix minus' on YT in the context of podcasting. I've tested it. It works great from a technical point of view.
However, it is HIGHLY inadvisable to implement it from a human behavior point of view. Even on very poorly attended live events, we would have multiple people call into an audio conference line patched into a teams live event who would leave outgoing audio on their phones un-muted after calling in because they were clueless about the etiquette of participating in multi-party audio conference calls.
So, we would get background noise from the phone going into the live event audio and causing havoc. The presenter would have to stop frequently and make endless appeals to people on the phone line to 'PLEASE MUTE OUTGOING AUDIO ON YOUR PHONES!!!!' which would basically be ignored so we would just have to hang up the phone.
If you consider that a live event can scale to 10,000 attendees, you can now understand that attendee call-in isn't officially supported for very good reason, The only reason I would consider doing it is to bring in one or two PRESENTERS by phone and then only after we have rehearsed with them prior to. Forget about bringing in attendees this way. If you're part of the DoD CVR Teams tenant, as I am, and you request an audio conferencing license for your account, there's no need even to set up a 'mix minus' for presenters. When you create your live event, it comes with an audio conference dial-in automatically attached that makes callers presenters when they dial in.
