Precise details of the outage weren't disclosed, but BT did explain that an engineer patched the platform that underpins Broadband Voice on Tuesday evening after many users suffered problems.
"Some calls were cut off every few minutes, and some people weren't able to make calls to the PSTN (public switched telephone network)," a BT representative explained.
Broadband Voice is a domestic service that lets users make telephone calls over the Internet, enabling them to turn their ADSL broadband service into a second phone line.
Unlike PC-based voice over Internet Protocol, or VoIP, services such as Skype and BT Communicator, which use a headset or a microphone, Broadband Voice uses a handset that plugs into the user's phone socket.
BT has about 22,000 subscribers for Broadband Voice, though the telephone company insists that "just a few thousand" were inconvenienced by the outage. The BT spokesman also added that such problems are not common.
It is technically possible to build a VoIP system with the same level of reliability as the PSTN network, but this incident underlines that the technology is more susceptible to downtime.
PC-based VoIP services are more likely to be unavailable than PSTN, as a power cut would render the computer inoperative. Standard phone lines are not affected by power cuts because they are powered from the local exchange.
Graeme Wearden of ZDNet UK reported from London.