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.
Such emergency services are provided by VoIP vendors in the United States by a system called Enhanced 911 (E911), based on the Wireless Communications and Public Safety Act of 1999. The VoIP E911 emergency-calling system associates a physical address with the calling party's telephone number. All VoIP providers that provide access to the public switched telephone network are required to implement E911, a service for which the subscriber may be charged. "VoIP providers may not allow customers to "opt-out" of 911 service."
VoIP service simply enables your phone calls to be transmitted and received over your home Internet connection instead of your telephone landline. This is perhaps why some people refer to the service as being an Internet phone service or broadband phone service. This does mean though that you need a high speed Internet service to be able to use VoIP for your phone service.