How can I encode a telephone number such that when a person chooses it, it starts a phone call. This would be analogous to a hypertext marker with the command "GO to URL" or "Message Client."
It would appear that one would use URI Scheme "tel":
message URL tel:+1-201-555-0123
But as Jeff's StackOverflow thread reference indicates, whether or not it works may be client-dependent. If it doesn't, and you control the client (device, browser), then provisional URI Scheme
callto: might be worth a test.
A further question arises as to whether the URI is delivered to the client via HTML/XML, or inside a PDF.
I've never needed to try any of it.
The reply from Bob Niland (Error 7103) is correct.
FrameMaker changed the plus sign ( + ) to an exclamation point ( ! ).
In the hypertext marker, I used the command Go to URL with the content message URL tel:!1-260-745-2907.
On my Samsung Galaxy J7 with Android ver 8.1.0 with the tel nbr encoded in a PDF file, when I choose the "hot" nbr, it ran the default app for phone. To call the nbr, I had to choose the green tel handset icon. This intermediate step provides a convenient opportunity to either stop the process or to continue it.