The implications of not having 911 service isn't the only issue facing Internet telephone users. If the power goes off, so does your computer -- and so does your access to phone service.

The solution for now, experts say, is planning a system with blend of old and new technology -- just in case. So, many residential customers are keeping their regular phone service as a primary line and signing up for VoIP to save on long-distance calls or to use their number outside their traditional area code.

"Power outages, although relatively rare in North America, certainly happen," admits Tracy Fleming, the national IP telephony practice leader with Avaya Canada, which designs, builds and manages communications networks for call centres and corporations across the country.

"Dial tone is not a feature, it's an expectation," he added, pointing to last year's big cross-border blackout as one reason why having a backup system is important for enterprise users. "A lot of people learned a lot of serious lessons. They needed to have [a] powered backup all the way down to the Ethernet switch if they're using VoIP phones."

While business VoIP networks may rely on so-called power-over-Ethernet technology to keep their systems running in a blackout, residential systems often do not.

Internet service providers have yet to develop a cost-effective way of transmitting power over broadband connections running into homes that would keep a VoIP adapter operating in a outage.

That means pretty much the only way a home user can keep a modem and VoIP adapter working is to have an uninterruptible power supply.

(When electricity is cut off, the Internet connection itself doesn't fail. High-speed cable and DSL broadband signals stay alive. But the devices attached have no power.)

But most home computer users don't have uninterruptible power supplies. Even if they do, a UPS only lasts for anywhere from minutes to perhaps an hour.

Residential VoIP service providers point out on their websites that power outages and network disruptions will render their modems and adapters useless. They insist customers acknowledge they understand what that means if they need to call 911 or any number, for that matter.

"It is a dilemma," admits Alex Suter, vice-president of marketing for Mediatrix Telecom, Inc., a Sherbrooke, Que.-based developer of VoIP access technology.

Last month , Mediatrix unveiled a new version of a residential VoIP gateway that recognizes the problem. The gateway doesn't require a UPS and automatically switches a VoIP phone to a traditional telephone network when the power goes out. That could save precious seconds in an emergency, when a caller isn't sure, for example, which of two lines is connected to the traditional telephone network and which is a VoIP service.

"You still have to have a regular telephone line," Mr. Suter said. "But by having a gateway which provides a bypass, you can at least use that POTS line to dial 911 and get calls out of there."