This is connected to a significant usability problem with `tel:` links: you have no way of knowing whether they’ll work, and if they don’t work, it could be in one of a few different ways. Maybe it’ll open a dialer app. Maybe it’ll do nothing at all. Maybe it’ll open an “unknown scheme” browser error page. Maybe it’ll prompt you to open it in an external app (I seem to have both Skype and Zoom willing to handle tel: links; neither is going to succeed). You largely can’t detect whether it has done something, might have done something, or has done nothing. Well, this article shows ways that you can detect likely results for some cases after trying it, but it’s not reliable and is depending on implementation details that are liable to change (especially since they’re a fingerprinting vector).

If it’s not going to work, I’d strongly prefer to not make the phone number a link—and perhaps even to present a different flow to the user (e.g. provide a form or mark an email address as the primary option). But if it is going to work, I definitely want it to be a link. It’s common to just guess from the user agent string or screen size whether it’s a mobile device, but that’s extremely flawed too—some tablets will and some won’t be able to dial, and even desktop platforms may well have some VoIP app.

Fingerprinting and usability are so often so significantly at odds. :-(