Can't be done in HTML alone-- and this is the html subforum. It can be done pretty straightforwardly with php or equivalent, making a dynamic page. I can remember at least one thread with the same question. That's assuming you're prepared to bar all caching of this page.
Option B is to, ahem, get an answering machine and trust to your readers' intelligence to figure out that a small business doesn't have people staffing the phones all day, every day. Even most huge businesses with technical support outsourced to a widely different time zone generally have published phone hours.