To make a link or a button action launch a phone call instead of linking to web page, simply put the telephone number in as the text of a button and add the number in as a link with a prefix of tel: , and where possible inboxes will make it a tappable phone link.
Note that the tel: and the phone number link must not have any gaps.
Tips:
When you add phone numbers that you want people to ‘tap’ to call you, make sure they are big enough for a thumb to tap without tapping anything else.
It’s also a good idea to make them the same colour as the rest of the links in your message or that section, so someone will know that they can tap it trigger an action.