Is it? They have almost infinite time and money, they can follow false leads and accusations almost indefinitely, and whoever makes that decision will never be met with any kind of consequence for making the wrong call. However sitting in a cell somewhere while they wait for you to give them information that doesn't exist definitely sounds like your problem not theirs.
Statistically you don't have to worry about this at all.
The vast majority of border crossings across the world don't involve any questions about phones anyway, but if you are at the stage where they do the characteristics of your phone or lack thereof can piss off the person who determines whether you are allowed to enter. And if you are at that stage entry denial will probably not be solely based on your phone or lack thereof because there was probably a reason they starting asking about it in the first place.
These aren't purely hypothetical scenarios but real life examples of things that happened to people I know.