The problem there is that VoIP traffic is much more sensitive to network bumps and potholes than most general office traffic. That translates to garbled conversations, 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 easy (though still there). For medium and larger networks, these tasks can be complex and time-consuming, which translates into added cost in terms of man-hours.
For larger systems, and for systems where security is critical for things like being compliant to vertical regulatory needs, your current internet connection might not be adequate. The internet doesn't do quality of service (QoS), and bandwidth can be unpredictable. Network congestion can ruin a conference call, and activities such as DNS hijacking can put your business and data at risk.

Callers aren’t going to spend time sitting through a menu that doesn’t let them, at the very least, leave a voicemail. If a caller knows their party’s extension number, an option can, and should, be “Press 1 to leave a voicemail,” or something along those lines. That way, clients can feel as if they’re expressing their concerns without needing to talk to an agent, and without the agent feeling sorry they couldn’t pick up while they were working on something else.
Forward calls to a pre-determined number during a power outage or when the connection to the cable modem is lost so you will never miss a call.