BT admitted on Friday that its Broadband Voice service crashed this week, leaving thousands of customers without their VoIP connection.
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," explained BT.
Broadband Voice is a domestic service which lets users make telephone calls over the Internet. It effectively allows them to turn their ADSL broadband service into a second phone line.
Unlike PC-based VoIP services such as Skype and BT Communicator, which use a headset or a microphone, Broadband Voice uses a handset which plugs into the user's phone socket.
BT has around 22,000 subscribers for Broadband Voice, although the telco 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 at present 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.