I remember back in the days of dial-up Internet access, it was difficult to have a regular call waiting service. The incoming call could cause problems with the modem connection. I also didn't like the idea of important callers getting a busy signal while I was online.
I found an Internet call waiting service that would automatically transfer incoming landline calls to a special toll-free number. They would receive voice mail message instructions and I would get a small pop-up notice that displayed the caller's phone number and replay the voice mail message.