Suppose we want to create a link for a contact which, when clicked on a smartphone, calls the contact. This can be accomplished with a link to a URL like tel:888-555-1234. Create such a link and add it to the ContactInformationTemplate.

If you want to use a [[double square brackets]] link rather than an a HTML element, you need to use ext, like this:

[ext[tel:888-555-1234]]

That's because tel:888-555-1234 by itself would be a valid name for a tiddler, too, so if TiddlyWiki isn't told it's an external link, it will assume it's an internal one. See the External Links heading here in the TiddlyWiki documentation for more information.