Example: in most cases, I don't have a microphone or headset on my office desktop PC (don't need it, we don't do zooms), have very slow internet access on my smartphone (forget OnAir calling with it, and anyway it would be too much friction to reopen the same web page on it just to call), but have a very well working landline phone nearby.
One possibility you might think about is to get a VoIP number terminated at your server (and possibly a free 1-800-xxxx but those don't necessarily work from abroad), where people can call, enter a code displayed by the OnAir client in the web page (like an extension ID, but it might be random if there's a value in obliging people to come to the web page before calling, e.g. to limit spam calling), and once done they'd be connected as if they were calling through the web page. The limitation with this solution here is that you'd need a number for each country you want to support, as international calls easily get expensive, especially from mobile.