The first time I did this exercise was in 2018 (when I first set up an XMPP server and Matrix Synapse for Communick) and there simply wasn't any working iOS app. Monal was the only app I found and managed to install for him. It did chat only and would crash. I do not recall to get e2ee working and the fact that it is optional made things confusing even for me - e.g, I wasn't able to switch between a desktop client and Conversations easily.
Element (then called riot.im) managed to do text, audio and video calls. The app had some bugs, but nothing that would block me from calling each other. The UX can still be confusing and I have occasional conversations where my father complains he can not hear me, most of them caused by my father not knowing that kept the video call but switched to the internal phone speaker instead of the external one.
I heard about Siskin some months ago. Honestly, I haven't tried it yet. It might be that is fully functional, but the UI is so bare that there is no way that I'll be able to convince my father to switch to it. He still complains that he'd rather use WhatsApp like everyone else, so whatever XMPP brings now will be a case of "too little, too late".
The problem here is not XMPP or Conversations but the closed nature of iOS which keeps apps like Conversations from being ported there. Apple does not like competition to iMessage or to its app store revenues so it fights tooth and nail to keep its precious as is now clearly on view in Europe with their ridiculous 'core technology fees' and other shenanigans.
Maybe you can give your father a non-iOS phone if that is what is keeping your experiment from succeeding? We're all on Android here, anything from stock Samsung like my mother uses to self-built LineageOS like I use and we have no problems like you describe. I video-chat daily with my mother without problems, we're using Jitsi Meet (hosted on the same server) for larger video meetings, we've used Nextcloud Talk (also hosted on that server-under-the-stairs) as well but now mostly use Conversations. Telegram also works well for video chat but that is neither self-hosted nor end-to-end encrypted so it is not a real comparison to Matrix or XMPP with OMEMO.
> Maybe you can give your father a non-iOS phone if that is what is keeping your experiment from succeeding?
That's a non-starter. He already had Android phones before, never liked them. O have to pick my battles, and getting him to call me Matrix instead of WhatsApp was already enough to call it success.
Besides, my point was less about the specific individual but the systemic issue. iOS is too large of a market segment to ignore, and I can not go around telling everyone "hey, why don't you just drop your shit Apple device and switch to something more open?"
Element (then called riot.im) managed to do text, audio and video calls. The app had some bugs, but nothing that would block me from calling each other. The UX can still be confusing and I have occasional conversations where my father complains he can not hear me, most of them caused by my father not knowing that kept the video call but switched to the internal phone speaker instead of the external one.
I heard about Siskin some months ago. Honestly, I haven't tried it yet. It might be that is fully functional, but the UI is so bare that there is no way that I'll be able to convince my father to switch to it. He still complains that he'd rather use WhatsApp like everyone else, so whatever XMPP brings now will be a case of "too little, too late".