A cable access network includes a cable trunk amplifier connected over a coaxial cable to A plurality of cable data modems to provide backup power to the cable modem during power outages experienced by either the trunk amplifier or the modems, thereby insuring uninterruptible IP telephone service. The trunk amplifier applies a DC voltage across the conductors of the coaxial cable. An AC/DC power converter and regulator and also a backup battery are sources of power for the DC voltage applied to the coaxial cable. A power outage sensor selectively connects either the AC/DC power converter and regulator or, alternately, the backup battery to the cable, depending on whether there is an outage sensed at the trunk amplifier. Each cable data modem connected to the trunk amplifier is partitioned into IP voice circuits and other circuits not contributing to the IP voice function. The other circuits include, for example, video/audio receiver circuits and IP data transceiver circuits. The IP voice circuits are connected to the subscriber's telephone hand set. The cable data modem applies a DC voltage to the IP voice circuits and the subscriber's telephone hand set. An AC/DC power converter and regulator in the modem and also the DC voltage from the coaxial cable are sources of power for the DC voltage applied to the IP voice circuits. A power outage sensor selectively connects the IP voice circuits to either the AC/DC power converter and regulator in the modem or, alternately, the DC voltage from the coaxial cable, depending on whether there is an outage sensed at the cable data modem. The other circuits such as the video/audio receiver circuits and IP data transceiver circuits are powered only by the AC/DC power converter and regulator in the cable data modem. In this manner, if there is a general power outage affecting both the trunk amplifier and many of the cable data modems connected to it, the power to be supplied by the backup battery at the trunk amplifier is only required to power the IP voice circuits and the subscriber's telephone hand set.