> stale stuff
No, not necessarily. "wtai:" is for mobile devices, where phone support is available by definition. "tel:" can work on PCs as well.
> they confused someone as the link did not resolve.
The functional problem is that as people have moved away from analog modems to broadband, fewer and fewer computers ship with voice modems. If there isn't a modem in the computer, then there may not be a driver to handle phone-related stuff. The browsers "pass" tel: URIs to the OS, and the OS recognizes that it's a telephone number, and looks for a device driver to handle it. If there is no such device (i.e. 56k modem or FAX/printer) attached, then the OS will return an error, and the user will get whatever the browser outputs as an error message -- "Some content on this webpage require a program that you don't have installed" from IE or similar from other browsers. Of course, this problem is not going to happen if the link is displayed on a mobile device.
So, if you use these types of URIs on "regular" Web sites, it's probably best to have "supporting information" surrounding the links -- Both in-context and as additional 'help' pages. Something like, "If your computer has a voice modem or an attached FAX printer, try our click to call!" -- With a bit of tweaking and the right audience, that could work.
I should add that I haven't tried using "tel:" URIs on Web pages in years -- since I removed my analog modem, in fact... :) So, some of the above may be inaccurate due to faded memory; Test, test, test if you try any of this.