Does it dial straight away?

No. Scanning opens the dialler with the number filled in and waits. Both iOS and Android require a deliberate tap before any call starts.

What happens to the 0 in +44 (0)20?

It is removed. That notation means dial the 0 domestically and leave it out from abroad, so keeping it would produce a number that does not connect.

Should I include the country code?

Yes if anyone might scan it from another country. A number without one only works for people already dialling inside the same country.

Phone Number QR Code Generator

Scanning brings up the dialler, number ready.

Phone number

Include the country code with a plus for numbers dialled from abroad.

Download as

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Scanning brings up the dialler, number ready.

How to use it

Type the number. Spaces, brackets and dashes are fine. They are stripped, because a dialler ignores them anyway.
Include the country code. Start with a plus and the country code if the code will be scanned by someone abroad.
Download it. PNG for screens, SVG for print. The tel value underneath shows exactly what will be dialled.

When you would use this

A phone QR code holds a tel link, so scanning it opens the dialler with the number already entered. It removes the step where someone reads a number off a van or a shop window and gets a digit wrong. The number is normalised before encoding: spaces, brackets and dashes are presentation only and are stripped, and a parenthesised trunk prefix like the 0 in +44 (0)20 is dropped when a country code is present, because that notation means dial the 0 only from inside the country. Nothing dials automatically. The dialler opens with the number ready and the person taps to call, which is both what the operating systems allow and what anyone would want.

Questions

Does it dial straight away?: No. Scanning opens the dialler with the number filled in and waits. Both iOS and Android require a deliberate tap before any call starts.
What happens to the 0 in +44 (0)20?: It is removed. That notation means dial the 0 domestically and leave it out from abroad, so keeping it would produce a number that does not connect.
Should I include the country code?: Yes if anyone might scan it from another country. A number without one only works for people already dialling inside the same country.