You can post code by using backticks (`) around your code to make it look like this:

<a href="tel:+99999999">Call Us</a>

or if you have a multi-line code block you surround the block with three backticks (```) at the start and end:

<a href="tel:99999999">
  Call Us
</a>

This also has the added benefit of using a mono-space font so that your code lines up correctly.