- no need to create an account
- mobile-friendly web app
I could be wrong but I'm pretty sure WebKit for iOS heavily restricts the capabilities of webapps, finding an application that meets any or all of this criteria for iOS users is unlikely in my opinion. I assumed you already considered Jami and Jitsi Meet but maybe you can consider Signal? It's not as mainstream as WhatsApp but it's still pretty popular. Depending on where you live and the type of people you're talking to, I'd say there's like a 5-20% chance some people you'll meet already have it.