If this becomes the norm, then I can see a startup offering "convincing" email, browser history, social media profiles, photos and telephone call lists to present to customs.
Give those pesky 5 eyes poison data of a fictional life via a 2nd phone.
one case I know about: tatoo artists travel the world for work as guests in different studios. most do so under tourism visa sice its only a couple months each place. their managers (and online forums) already have a list of things that can be in the luggage and in their phones and instagram feeds so that they really do look like tourists.
The problem with this approach is that just like a cover used by a spy, you'd have to memorize all the details. One slip would blow this phony profile, then you'd be in jail for making false statements to a govt. agent...
I have my travel phone which I pop a SIM card into when I land, and I have my normal phone which I leave at home. I don't do it for privacy reasons, but just in case I lose a phone, I'd rather lose my crappy Motorola with a few contacts in it and a few chat apps, than my iPhone X with my entire life in it...
That ultimately doesn't matter. You tell them it's a travel phone, because you fear losing your expensive regular phone. Your social media accounts have elaborate, long passwords that would be impossible for a person to remember and are stored in a password manager at home (countless formulations you can go with in this story).
At that point there's nothing else they can get from you, the odds are overwhelming that'll be the end of the discussion. There's probably a one in a million chance that eg the TSA might detain you for a few hours out of spite if they think you're screwing with them.
Give those pesky 5 eyes poison data of a fictional life via a 2nd phone.