'tel' is the short written form of 'telephone number'. You will almost never hear it said out loud — it lives on paper and on screens, in the places where space is tight and everyone already knows what is coming: business cards, letterheads, shop signs, adverts, the contact box at the bottom of a form.

It is normally written with a full stop after it, 'Tel.', because it is a shortening of a longer word, and often with a capital T at the start of a line. Then a colon or just a space, then the number: 'Tel: 02-2345 6789'. On the same card you will usually see its companions — Fax, Mob (mobile), Add or Addr (address), Email.

When you read it aloud, you say the whole thing: 'Tel. 02-2345 6789' is read as 'telephone oh two…' or, more naturally these days, 'the number is…'. Nobody says 'tel'.

It feels slightly formal and slightly old-fashioned now. It belongs to the era of the printed business card and the phone book. Younger people making a website or an Instagram page are more likely to write a phone icon, or just 'Phone:', or nothing at all. But it is still completely standard on official forms, letterheads and shopfronts, and you will meet it constantly on business cards in Taiwan.