From Newsletter 43:

You can mark up a phone number as an anchor link. It involves using a protocol, like http:, only it's tel:. So...

<a href="tel:555-555-5555">555-555-5555</a>

What I don't fully understand is what the perfect way to use this.

• iOS automatically detects phone numbers and makes them into links, so you don't need to do anything. Android doesn't, though.
• Do desktop users actually like tel: links? Most desktop computers aren't capable of making telephone calls. If they are (like you're using a Mac with Safari and have Facetime set up which pairs to your iPhone), how common and desireable is that? Skype can make calls on desktop too, but that has it's own protocol.

It seems like the best practice would then be like (pseudo code):

IF (android && actually_a_phone)
   OR (desktop && actually_set_up_to_do_calls) {

  // inject tel: links

} ELSE {

  // leave phone numbers alone

}