Just like Excel seeing everything as a date, mobile browsers automatically interpret many numbers as telephone numbers. When detected, mobile browsers replace the text in the HTML with a clickable “<a href=”tel:...”>” element that will call the telephone number when selected.

The article has live test-cases for numbers that may be detected as phone numbers. For example, “5555555555555” is detected as a phone number but “2222222222222” isn’t? Weird.

Try reading on your phone: https://sethmlarson.dev/mobile-browsers-and-telephone-numbers