1-800-555-1212 Call Me...

Meta tag...

Add this meta tag to your page head, if one does not already exist:

<meta name="viewport" content="width=device-width">

Markup:

<a href="tel:18005551212"><i>1-800-555-1212</i> <b>Call Me...</b></a>

CSS...

a[href^=tel] {
display: inline;
text-decoration: none;
color: #000;
}
a[href^=tel] i {
font-style: normal;
display: block;
}
a[href^=tel] b {
font-weight: normal;
display: none;
}

/*Media Query - Adjust as necessary to target the devices you want to */

@media (max-width: 480px) {
a[href^=tel] {
display: block;
background-color: #E5E0B9;
padding: 1em;
box-shadow: inset 0px 16px 6px rgba(255,255,255,.3);
border-radius: 5px;
text-decoration: none;
color: #000;
font-size: 130%;
text-align: center;
}
a[href^=tel] i {
font-style: normal;
display: none;
}
a[href^=tel] b {
font-weight: normal;
display: block;
}
}