Hi,

To do that, please do follow steps below:

1/ Go to Appearance > Customizer AMP ( not Customize ) > Header Options > Add Custom Codes After <body> Tag > and add more this code:

<a class="penci-call-now" href="tel:999999999"><i class="fa fa-phone"></i> CALL NOW</a>

Note: Replace  number  999999999  on the above code to your phone number.

2/ Add more this code to Appearance > Customizer AMP ( not Customize ) > Additional CSS:

body{ padding-bottom: 50px; }
a.penci-call-now, a.penci-call-now:hover{ display: block; text-align: center; font-size: 24px; padding: 0; height: 50px; line-height: 50px; background: #0e47a1; color: #fff; font-weight: bold; position: fixed; left: 0; right: 0; bottom: 0; z-index: 10; }

Hope this helps,

Best Regards,

PenciDesign