This person is so paranoid, that they believe that a cyberweapon developed by a private company in Israel that uses previously-unknown bugs in the most sandboxed messaging system you can get on a phone are going to be deployed against them, so they should not use the calling, texting, or any other "phone-like" functionalities of a phone.
They then distrust that the End to End Encryption is in-fact End to End, and then think that using Signal or something is more secure, when if a bug in a system more sandboxed than Signal was found (iMessage, which has BlastDoor which Signal does not have), it is more than likely that Signal has it's own zero-days in it, so you shouldn't be using that either.
That's paranoid, and if you are that paranoid (which, maybe you have a reason to be), your solution isn't well thought-through. You shouldn't be using a phone if you can help it.