Louis Rossmann did a video on a cell company that doesn't require things like an address or CC to get a phone/phone number about a month ago. It was posted here by someone, I was going to post it but someone beat me to it.
Rules from someone I know that works in some very interesting places. 1. Reboot phone daily. 2. Change your pin at least once a month, require verification. 3. Set all messages to auto-expire/delete.
Those are the minimum things you should be doing even if using signal.
There are zero-day exploits against ios/android. Even if there weren't, the baseband/cellular modem, enclave, boot rom, and wi-fi hardware are all compromised beneath the OS. Signal is good for dragnet surveillance only. If you are a target, you need to stay off your sail foam unless you soldered it together yourself at your garage chip fab and then installed the OS you wrote yourself.