How to assign a DIV to a different class when viewing on mobile?

Question

css

I mean not to put a completely different css on mobile devices, but only for one DIV so that the classes change, i.e. how to correctly write media inside html page? I hope I made it clear.) Thanks
Desktop visitor:

-----------------
<div class="desktop">
<p>000-000-000</p>
</div>
----------------------------

Mobile visitor:

---------------
<div class="mobile">
<p>000-000-000</p>
</div>
----------------