How to add a button image for tap to call in a responsive theme

I have the code for adding tap to call:

Tap To Call

Instead of reading “tap to call” I want
to add a button image that has the
phone number written on it….. because,
this is a responsive theme and, if
someone accesses it on a PC, they won’t
see the number…

how do I add the button image to this
code???