How prominent should it be?
In most cases, having the phone number in the burger menu and the footer is enough.
However, depending on your store, if you want your users to just pick up the phone and call you, then that call to action can be made sticky (a bar at the top or bottom of the screen), or prominent (with a phone icon).
Mobile option 1
Phone # in promo bar:
Mobile option 2
Phone button in main header row:
Mobile option 3
A sticky button at the top of bottom of screen: