What the code contains
A tel: link, which is what sits behind every clickable phone number on the web:
tel:+491701234567That is the whole payload. Because it is so short, the resulting code is one of the simplest you can make — chunky modules, reliable from a small print, forgiving of bad light and awkward angles. It is a good code to put somewhere difficult.
It opens the dialler. It does not dial. Every phone stops and waits for a deliberate tap, and any that did not would be a security problem.
International format, always
Write the number with the country code and a leading plus:
- Germany:
0170 1234567→+49 170 1234567 - UK:
07700 900123→+44 7700 900123 - France:
06 12 34 56 78→+33 6 12 34 56 78
A local-format number works perfectly for people whose phone is already on your network and fails for everyone else, with no error and no explanation. Since the whole point is removing friction for strangers, the international form is the only one worth encoding.
Where a short code beats a long one
This is the practical argument for a phone code over almost any other type. Because the payload is a dozen characters, the pattern stays coarse, and a coarse pattern reads from further away, in worse light, at a sharper angle, off a rougher surface.
That makes it the right code for the awkward places:
- Vehicle livery — read from across a car park, at whatever angle the van is parked.
- Site boards and scaffolding — outdoors, weathered, seen in passing.
- Machinery and equipment — small labels on oily surfaces.
- Estate agent boards — read from a pavement, from a car, in the rain.
- Emergency and out-of-hours notices — where the person reading is already stressed.
Think about who answers
A printed code outlives the situation that prompted it. A number on a site board is still there when the job finishes; a number on a vehicle is still there when the vehicle is sold. Point it at a number that will still be monitored, and prefer a business line to a personal mobile for anything that goes outdoors and stays.
Print the number too
Not everyone scans. Some people would rather read a number and dial it, some cameras will not parse tel:, and a scan on a laptop often does nothing at all. The written number costs one line and covers every one of those.