Make phone calls from a Drupal site
You could use the tel: or callto: href identifiers depending on the method of your phone call (Skype uses callto:, mobiles that support this feature will use tel:.  

For example, if the number is 123-456-789, your link would look like one of the following:

&lt;a href="callto:123-456-789"&gt;Call this user&lt;/a&gt;


or

&lt;a href="tel:123-456-789"&gt;Call this user&lt;/a&gt;


Check this (old) blog post for some examples/ideas on implementation.