Posting this here because it appears to be the Horse's Mouth:
(Does not answer the question but is a useful link; note in particular that "visual separators" are optional.)
I assume you started out with the same random searches I did, generally leading to the unhelpful suggestion that you might include user-agent detection. Which kinda defeats the purpose of having a phone number in the raw html :(
Does your 404 page act as if you had simply typed in the URL
If so, you should be able to make a rule to intercept requests beginning in "tel" and send them to a custom page. The rule will only be seen by devices that don't recognize "tel"; it won't interfere with telephone users.