Voice over IP (VoIP) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.
VoIP completely relies on internet connection. It means that VoIP cannot be completely relied in places where internet connection and broadband speed are fluctuating. And probably during most of the natural calamities internet connection is shut down and this makes VoIP useless at emergency time. This is one main disadvantages of VoIP.
A VoIP is through the internet it is prone to cyber attacks like hacking and other threats like viruses, worms, bugs etc. A conversation through VoIP can easily be accessed by a third party by means of hacking.
Most of the time when you call an emergency call like 911 the help arrives by tracking your location. But as VoIP is just a transfer of data between two IPs it cannot be tracked. This makes emergency calls from a VoIP less helpful than usual call.
The voice clarity in a VoIP is relied on the internet speed. Even when the network connection is pretty good the voice clarity in VoIP can be really low.
The facility of VoIP is unavailable at the times of a power shortage. This makes VoIP a bad option in time of a power failure. At this time a usual phone is always a better option than VoIP.