[qwerty456127]

You have to spell your ID to people so they can contact you anyway and a number is the easiest to spell aloud when communicating to people from different countries because everybody (every language) calls the same letters a different way and almost nobody cares to study proper letter names as this is the most useless knowledge about a language otherwise.

[EGreg]

No need to spell it

Send a QR code or a link that can be used only once.

Why have an identifier that any number of people can use to contact you?

[qwerty456127]

> Send a QR code or a link that can be used only once.

I most often have to spell my contact details in a voice phone call because my primary job is to communicate to live people all over the world, not to code. Believe it or not but people actually call my office desk phone regularly (although I always prefer email if possible). Even in the IT sector (let alone administrative tasks, healthcare, utilities, etc), whenever you need a rack in a datacenter, new servers or whatever you often are meant to submit your phone number on their website and then they call you. Some very big Internet and datacenter operators don't advertise any ways to contact them other than by phone, some would publish an email or a contact form but ignore you until you call them.

> Why have an identifier that any number of people can use to contact you?

The same as the above.

[EGreg]

Well then you are a special case different than most. You might not need the privacy we’re talking about here. You’re giving out ways to contact you that, if abused, will result in irreparable spam to that account. But it’s just part of a job you do. So that’s fine.