Looking into US privacy-respecting alternatives to iPhone this weekend, both T-Mobile and Apple are imminently forcing VoLTE. I've not yet found reputable Android AOSP-derived ROMs or other open source firmware supporting that. (Also, carriers reportedly will allow/deny devices by IMEI model.) Looks like regulatory authorities are OK with all this.
Stock Android installs on devices carriers will permit are more respectful than iPhone in some regards, but are still very heavy on commercial privacy invasion and security vulnerabilities. They'll also probably get abandoned for security updates, etc., and alternative ROMs no longer mitigating that.
Librem 5 looks good in principle, but the only Librem 5 on eBay just now was a preproduction one for $700+, and dunno how long Purism will be wrangling carriers.
The few featurephones seem to be crap and/or include integrations with the most infamous social media services, which is a really bad sign.
None of the various Linux phone efforts seem ready even today, and carriers are a moving target.
I just spent the weekend trying to move from my iPhone. (And, when I tried a SIM in a stock Android phone, the carrier suspended my service, taking many hours of misery to get it restored in the iPhone it had been in.)
How do we keep even a minimal phone number (voice & SMS), without all these privacy&security regressions? (Practically, and solid, and through at least the 2021-2022 VoLTE switchover, and hopefully years after. A phone number is still required by some gov't and critical services. Wireless data would be nice, but we currently could survive with WiFi, etc.)
CalyxOS as a special mention.