Hi,

You can do this anywhere with HTML. You can add it to revolution slider as well.

In the backend revolution slider editor, the button might not look the same as it will look on the front end. 

The basic HTML for such link is:

<a href="tel:1-907-826-2273">Call now!</a>

You can add classes to it, so it looks like a button:

<a href="tel:1-907-826-2273" class="btn btn-c2 btn-flat btn-md btn-round">Call now!</a>

And finally, if you want the phone icon as well:

<a href="tel:1-907-826-2273" class="btn btn-c2 btn-flat btn-md btn-round"><i class="fa fa-phone" aria-hidden="true"></i> Call now!</a>

Any of the above will do the job.

Andrej