In order to add "Click to Call" icon on your mobile website, first make sure that you have added the phone number of your business. This can be done under the Phone section of the company information form on the website settings page.
Click on the gear icon and then click on the website settings menu. Finally, check that the phone number has been added.
If you have added the phone number in website settings correctly then the "Click to Call" icon will appear on your mobile site automatically.
To learn more about adding a phone number to website settings click here.
To learn more about website settings click here.