When I'm travelling I only have Internet via my iPhone, which I can share to my Mac via Personal Hotspot just fine. However, I have a VoIP desk phone which only gets internet via ethernet cabling. I have tried plugging the phone directly in to my Mac, and then turning on Internet Sharing to share the connection from Wi-Fi to the ethernet ports. But the phone does not pick up any connection. Is this possible and how can it be done?
I've had a lot of experience using internet sharing under OS X. For inexplicable reasons, it's quite unreliable. Internet sharing also jumps around between OS X versions. What works in 10.5 might not work in 10.6 which might not work in 10.7, all the way though 10.10. Your trick of putting a switch in between is a good hack.
You could also try using mobile router as an access point. It would be a client to your phone's hotspot and then offer ethernet to both your VOIP and your Mac. This offers the benefit of being able to make calls with your Mac either absent or turned off.