In the typical office of today you will find a coffee machine (espresso, please), staplers, paper, computers, and the more important things like an internet connection and a phone line. Neither one of these you can do without. Which one would you rather make redundant, the phone line or the internet line ?
Well, that all depends on the type of business you're running. I know someone who is running a nice little business using almost exclusively phones.
Big advantage : By also using yout mobile phone youre always reachable.
Disadvantage : dependency of the phone(s) and cost.
I would make my internet line redundant before I would ever think of the phone. How come ? Its simple. I do all of my business online – from consultancy jobs to selling stuff – all is conducted online. The buggest advantage is probably the fact that most of my sales don't need much attention as it is all automated. In conultancy there are the occasional phonecalls, but it's definetively not much. And about my phone line – its a voip line that runs over my internet line. My office doesn't even have a traditional phone line….