Telephone links are a thing. Like an anchor link you tap to (probably) go to another page, these are links you tap to call a number on a phone-capable device. They’ve been around for quite some time. Yet, they cause me a lot of confusion. For example, many devices will automagically recognize phone numbers and do the linking for us, but not always.
There is enough web traffic on mobile devices and plenty of desktops apps that are capable of making calls, that it’s worth knowing more about phone links.
A nice guide on how to use HTML telephone links. Details current browser support and how to use them in structured data for inclusion in Google’s local business listings.