So ...

Option #1: Adding a click-to-call feature manually

First of all, you need to understand that basically we are talking about a "regular" hyperlink.

On Android and iPhone devices these links will open the according phone application.

On desktop, Windows 10 will provide an option to open it in Skype, and Mac users will see an option to oen it in Facetime.

With that having said, let's see how to create these links ...

The link itself, in the form of a regular text link looks like this:

<a href="tel:+12223333333">Call Me!</a>

Now let's use HTML to add an image ...

Obviously, the first thing, is to upload the given image in your media library.

Go to Media -> Add New and upload the image you want to use. Then, you need to click on the "Edit" link ...

... to access the "Edit Media" window where you need to copy the file URL (in order to use it in the next step):

Now you are ready to insert the image and the link wherever you want.

All you have to do, is to edit the given post, page or custom HTML widget where you want to display the click-to-call feature.

I'll use the post text editor to add the image and the phone link next to my text:

And here's the HTML code you need to use:

Of course, you'll have to edit the phone number, the domain name, the image name and the alt text according to your needs.

And here's the result:

And as I said, once clicked, the link will allow you to pick a phone application:

In my case (Windows 10) it would be Skype:

COOL!