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.
Emails cannot track telephone number links because they do not redirect someone to a new web page, they are local links which trigger an app that is part of the device’s operating system or taken over by another app’s add-on, for example Skype’s browser add-on which looks for phone numbers in a page.
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.