The problem there is that VoIP traffic is much more sensitive to network bumps and potholes than most general office traffic. That translates to conversations breaking up or cutting out entirely, difficulty connecting over Wi-Fi, or (worst case) dropped and lost calls. If your business is small and your network is essentially contained in one or two wireless routers, then your configuration and testing headaches might be fairly small (though still there). But for medium and larger networks, these tasks can not only be complex, but also time consuming, which translates into added cost in terms of man-hours.
E911 (Enhanced 911) is a little different to traditional 911 services and is something that you should be aware of. With E911 you need to provide your home address, and this information gets passed to the emergency services if you dial 911. The concerns are that if you have a power outage or an Internet outage then you will not be able to dial 911. If you have a cell phone then you can dial 911 from your device. If you are ONLY reliant on VoIP for your phone service you should be aware of this situation.
From an end user point of view, the actual phone service works in the same way, you pick up the phone to answer a call or to dial a number just like with a landline service. Number porting means you can keep your existing phone number and simply switch it over to your new service provider. The residential VoIP providers take care of the call routing and you can call any destination and receive calls from anyone, just like with regular home phone service.