fixed

.mobile{
	position:fixed;
	bottom:20px;
	right:20px;
	width:50px;
	height:50px;
	background-size:cover;
	z-index:999;
	background:darkblue;
	border-radius:50%;
	padding:8px;
	opacity:0.6;
}

<a class="mobile" href="tel:xxxxxxxxxxx">
	<img src="mobile.png" width="34" height="34"/>
</a>