Official comment
You can now use tel: links anywhere you create a link. In the link editor, select "TEL" from the protocol drop down:

Allowing people to one-click their way to our phones has converted like CRAZY...this is pretty important to our funnel. We can create the "tel" formatted links in an HTML editor, but not in the ONTRApages dialog boxes.
We cannot use tel://8055551212 in the URL field. We have found that you allow the similar mailto://user@domain.com string to create a "mail to" link. Please add the ability to create a tel:// link in accordance with RFC 3966: http://www.ietf.org/rfc/rfc3966.txt
You can now use tel: links anywhere you create a link. In the link editor, select "TEL" from the protocol drop down:

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Here are some images ... first, the allowed "mailto" link format:

Here is the error when trying to create a tel:// link:

Thanks for putting this in, Frank!
Agreed! Please support!
Hi Frank. Is it possible to use tel: links when the phone number is a merge field?