For two years, my windshield had a laminated card: "In case of blocking, call 98XXXXXXXX."
It made sense. I park in tight lanes — someone always needs to move my car. A number on the dashboard solves that.
It also created a problem I didn't see coming until the spam started.

The problem wasn't the sticker. It was the number.
Once your personal mobile is sitting on a windshield in a public lot, it isn't private anymore. Anyone can copy it. I started getting loan spam, a late-night "saw your car" message, random WhatsApp adds.

And I couldn't take it back. Every photo someone snapped of my dashboard still had my number on it. That's the part nobody talks about: a contact detail printed into the real world never expires.

What I actually wanted
A way for a stranger to reach me about my car without ever seeing my real number.

So I built a small thing: a QR sticker for the rear windshield.

Stranger scans QR
→ opens vehicle profile (no number shown)
→ taps "Call" or "WhatsApp Owner"
→ routed through a proxy
→ owner's real number stays hidden

Change the destination number anytime. The sticker never changes.

The privacy layer was the real work — not the QR
Generating a QR is three lines. The actual engineering was the masked contact routing:

Call masking: the button dials a proxy that forwards to me. The
scanner sees the proxy, never my SIM.
WhatsApp first: in India nobody wants a cold call about a parked car — they want to fire off a quick "gaadi hata do" message. So WhatsApp had to be primary, masking on top.
Revocable: sell the car, detach the profile. The contact was never baked into the physical tag.

If you've built anything with personal data in India, the lesson is familiar: **privacy can't be bolted on later. It has to be the foundation. **The moment "hide the number" became the core instead of an add-on, the whole design got simpler.

India-specific things I had to get right

WhatsApp is the channel, call is secondary. Build the masked WhatsApp flow first or you've built the wrong product.
Monsoon-proofing: UV-resistant vinyl + laminate coat, QR recessed so the print survives sun and rain.
No app for the scanner. The person scanning your car will never download anything. QR opens in any default camera; budget Androids mostly don't do NFC.

What I'd do differently
Build the privacy layer first — before the profile UI, before the sticker. I made the page look nice and retrofitted masking, which is backwards. In India, the privacy layer is the product.
I eventually folded this into ProfileTap's vehicle profile, since the same masked-contact idea applies to pets, luggage, and family tags too — anywhere a stranger needs to reach you without owning your number forever.

If you've shipped anything that puts contact data into the physical world — a tag, sticker, NFC card — how did you handle the "this is now permanent and public" problem? Curious how others think about it for the Indian market.