When it comes to business, getting VOIP is going to be a hands down Linux affair. Because most Linux based systems are made to function as both desktops and servers, the ability is there to scale phones and have multiple VOIP lines running off of the system, especially as most companies need multiple phone numbers to function properly. If you want to get that done, then try a piece of software like Asterisk that makes a Linux based PC into a VOIP server.