It is possible to code a link for a phone number in HTML - however, this isn't always recognized. (Skype and devices tied to certain VOIPs in particular will need a separate code.)
In general, the HTML code reads as follows:
Email with phone access and mobile devices:
<a href="tel:5551234567">Call (555)123-4567</a>
For Skype and computers without phone permissions:
<a href="callto:5551234567">Call (555)123-4567</a>
Keep in mind, this is only useful for local (not international) callers.