Your boss gets paid to do it. It’s in the employee handbook.
Also, do they pay a part of your phone bill? Do you have access to a company phone? If they would clock you in, could you tell them you’ll do it when you get back to site? What if you get injured while calling your fill in? What if you get injured, then initiated the conversation about filling in, then claiming the injury happened on the clock? Infinite free catastrophic insurance hack?