Mobile responsive banner CSHTML

I have a small banner on my website, looks good on the web version, but on mobile, it is kind of messed up. I was planning to use the @media function in style tag, I'm still new to CSHTML and I notice it does not have the style tag. Here's the code;

<div style="width: 100%; background: #AA0114; color: #fff; font-weight: bold; padding: 20px 10px;">
    <span style="font-size: 18px;padding-left: 5px;">
        Contact us for more information. Call us at <a href="https://wa.me/60100000000" target="_blank"
            style="text-decoration: underline;">60100000000</a></span>
    <span style="float: right;font-size: 18px;padding-right: 20px;">E-MAIL: ouremail.com.my</span>
</div>