At Admin Panel -> Settings -> Switches, there's a "phone numbers" switch. When enabled, this prompts listing submitters to supply a contact phone number while submitting. The phone number is then disabled on the listing details page so that people can call the business.

If you'd like to let people call businesses directly through your site, you may want to integrate a click-to-call widget like tringme or google voice. Feel free to discuss this with WSN support.