As the lines continue to blur between various technologies, it is almost not fair to refer to modern, unified communications systems as 'phones' or 'phone systems,' but we have to call them something, right?
If a solid Internet connection is in place, or one can be had for something less than an arm and a leg, we generally start here and reverse engineer. Reasons are legion.
There are really fantastic on-premise phone systems available for a price...and then another price for ongoing care and feeding. And there are still many situations where an on-premise solution is the best and only choice and we will make that recommendation. Mostly, though, our clients are headed in exactly the opposite direction and not interested in a big spend to bring gear in-house.