I see it differently. If you're running a business that is open from 9-5, answer the phone. In this day and age, it's pretty simple to link a business phone to whatever phone you're walking around with, even if you're doing repairs in the park or helping a camper at their campsite. At the end of the day, switch it back to the answering machine to handle after hour calls. When you do, take the time to review them a few minutes before you open for business.
On occasion we'll be on the road and select a couple of campgrounds from RV Park Reviews, the night before and then call them the next morning while driving, to see if they have room for the night. It's frustrating to get an answering machine and then have to wait hours for a call back. Sure, you can plan ahead, but often when just travelling, you don't always know how far you'll drive that day.