Conventional phones are connected directly to telephone company phone lines, which in the event of a power failure are kept functioning by backup generators or batteries located at the telephone exchange.
However, IP Phones and the IP infrastructure connect to (routers and servers), which typically depend on the availability of mains electricity or another locally generated power source. Therefore, most VoIP networks and the supporting routers and servers are also on widely available and relatively inexpensive uninterrupted power supply (UPS) systems to maintain electricity during a power outage for a predetermined length of time.
The amount of time typically ranges from as little as an hour and up from there, depending on the quality of the UPS unit and the power draw and characteristics of the communications equipment.
Voice travels over the Internet in packets in the same manner as data. So when you talk over an IP network your conversation is broken up into small packets. These voice and data packets travel over the same network with a fixed bandwidth. This system is more prone to congestion and DoS attacks than traditional circuit switched systems.