It's been a while since I tried using my phone outside the house in Airplane Mode but with wi-fi (and Wi-fi Calling) turned on. I tried it at work a few years back when their T-Mobile reception was terrible, thus causing my battery to drain more quickly, and I'm pretty sure I could text people (this was before RCS was a thing). So I'm not sure that you can only text other RCS people on wi-fi nowadays. @Kizzy Catwoman, do you know for certain that regular non-RCS SMS won't go through if the phone is on Airplane Mode, but Wi-fi Calling is on?

BTW, the experiment with using wi-fi only at work only lasted a couple of days, because the wi-fi at my work is also lousy, and since I'm constantly roaming around, I'd keep hitting weak or dead spots, which would make my call drop. @J Dubbs, this is probably the reason why some people have issues when using wi-fi calling -- if the wi-fi network is wonky, but the phone is set to prefer calling over wi-fi, then it might encounter issues. There must be a threshold where the phone decides the signal is too poor and switches over to mobile data, but I don't think there's a way to adjust that threshold. So what I do is to turn on wi-fi calling, but I set it to prefer cell signal.

I'm not sure about the VoLTE issue. Do people say that you recurrently cut out, or is it just intermittent? If it's only intermittent, it wouldn't by chance be related to call waiting, would it? I know that AT&T has historically been annoying about compatibility issues with BYOP devices, so this may just be another one of those things. But I think you need to keep VoLTE on, because AT&T has not only shut down its 2G network, but will be shutting down 3G soon as well: https://www.xda-developers.com/t-mobile-att-require-volte-phone-calls-shut-down-3g/

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