Leaving your number under the wiper solves one problem and creates another. The person you blocked in really can call you — but in the meantime your number is visible to everyone who walks past the car.
What actually happens when someone scans the code
Scanning opens a web page, not an app. The person scanning does not have to install anything to tell you that you left your lights on or that you are blocking an exit. They pick one of the prepared messages and it reaches you in Faar.
Your number never appears. The conversation goes through the app, and the other side only sees that someone owner-side picked it up — not who you are.
When a prepared message is not enough
The messages someone can send are deliberately prepared in advance. An open channel for free text, with no barrier and no app installed, would be an invitation to abuse — and the prepared messages cover very nearly everything that actually happens in a car park.
What is free text is the status you put on your own vehicle: a short note such as “Back in 10 minutes”, visible to anyone who scans the sticker, along with a countdown of how much longer it holds. You choose the duration, and when it expires the status disappears.
If you do want to show your number
Hidden is the default. If a situation comes up where you would rather the other side could call you directly, you can turn that on — but it is your decision, not a setting someone inherits.