Please open the header.php file and find the header-phone.. remove the image assisted-living.png file and instead paste the code below to render a css button inside the same div which contained the now removed image. You should change the link in the code to reflect the correct destination url within your site and style it as you see fit
Here is the code for the button
<a href="http://www.my-url.com/change-this/to-whatever-you-want/" class="avia-button grey custom large" style="background-color:#3a5b43;border-color:#000"><span class="avia-">Tell Us How We Are Doing</span></a>
As for "call function url", I am not sure what that is and neither does google it seems. As I don't own a mobile, please show me a website and point to a phone number on it that is a "call function url", and I can replicate it.