In the end of 2016 Facebook have rolled out new type of ads which give us a possibility to create a post with "call now" button. First you need to setup your phone number in your page settings. Then press "Get phone calls" button
near the place for new post box

Write the post with attractive content determine your offer and find prominent heading. Publish your post.

Use "Boost post" button to promote your post and get phone calls from your potential customers.