By default, Jitsi does not allow - or at least used to not allow - video conferences through the standard mobile browser on mobile devices. Instead, it is designed to work through the native Jitsi mobile app (free) or another native mobile application, to avoid various types of issues.

You should have seen a warning about this when you tried to start a video conference in the browser on the iPhone.

In the current configuration, this means you can only start video conferences reliably through the web version on desktops/laptops, unless you use the mobile app.