I think it's important to understand the domain to know if those edge cases are likely to happen. If no one on payroll is ever on call, then no need to design for that. Solution works as intended. If it turns out we need a more robust calculator later, then we can design for that. But adding that complexity before the domain requires it seems unnecessary to me.

But also, just because there is complexity in the domain doesn't mean there needs to be complexity in the software. An elegant, simple solution could be implemented for calculating payroll or adding infinity. That's the hard part though.