If you're really only making phone calls, the built in location tracking is probably the biggest issue? AFAIK, you can only use an off the shelf iPhone with an Apple account, and a similar Android phone with a Google account, so your location will be tied to and referenced with those.

Apple have branded themselves as guardians of their users' data, so many consider that a safe assurance. YMMV but it may be slightly better than Google's Dodgier approach. When in doubt, go to settings and turn everything off you don't use, location services foremost.

You may want to disable other apps that come with your phone as well. Basically anything you don't use. I don't know how much data can be harvested from background services of an app that doesn't have a user signed in, but at this point I'd err on the side of caution. Plus, as you say, your position can always be approximated by your mobile carrier through the cell towers you're connected to, but that goes for dumb phones as well.

Personally, I only use Android smartphones with custom ROMs like LineageOS without installing the Google apps or services framework because I Just Don't Use Google. Instead I install microG to spoof the GSF to apps that require it. That's a privacy compromise I can live with because I use my phone as an internet device as well. Needless to say I take privacy precautions on an app level as well.