At one level, you're toast, right? You need a burner phone you bought with cash, without using ID, and to activate it without linking it to your person. You need to never have it with you at your commons places to be (house, work, coffeeshop on the corner, etc.) - and once you start talking using apps on a smartphone, you've multiplied the complications here 1000x. If you care that much, you probably should just give up on cell phones.
But, there are a tons of ways to make your usage of cell phones safer and more secure. The Guardian Project is a great place to start - https://guardianproject.info/apps/ - you can get their apps from the Play store, from the F-Droid OSS repo, or as APKs directly. It brings Tor to your Android, OTR chatting, end-to-end encrypted VOIP calls, and even PGP email.
iOS is a bit further behind with all of this, for various reasons.
There was a great guide on this last year, but the site seems to have gone offline. Some intrepid data-rescuers have put the content up on github: