The issues surrounding BT's switch to the internet protocol for its entire infrastructure are epitomised in a new low-cost router from Zoom/Hayes that includes a Voice-over-IP (VoIP) port allowing you to use a standard phone to make free or cheap calls over the internet.

A hotkey allows you to switch between VoIP and standard phone operation, mirroring the choice subscribers will face when they get the 'broadband dialtone' of what BT is calling its 21st Century Network (21CN).

Users who wish to use both broadband and the old phone system will see little difference from the current ADSL setup, as they will still need to install filters to separate the signals. But even BT expects to see a major switch to VoIP; what is not clear is how this will work.

On present pricing structures, pure VoIP calls are free apart from the cost of web access. They have not gone mainstream because VoIP calls can be tricky to set up and quality can be erratic.

IPv4, the dominant IP version today, has no quality of service provision to ensure the timely arrival of data packets necessary for voice calls.

This can result in distortion and delays so that at worst a VoIP call can sound like Nasa space control calling the moon.

But call quality can also be very good and it has improved with the shift to broadband, better compression, and better hardware - the new Zoom/Hayes X5v VoIP router has a large buffer to smooth data flow.

The quality should get more reliable still with the rollout of 21CN which, with an increasing proportion of the wider internet, will use next-generation IPv6 which can guarantee bandwidth to time-sensitive traffic.

But service providers may charge extra for quality of service to gain revenues from voice traffic, if they can persuade users that it's worth paying for.

In the meantime a sudden major switch to VoIP could hit BT with a massive revenue loss. The company does not believe this will happen; but it bowed to the inevitable earlier this year by announcing VoIP services of its own, which it now turns out was just a prelude to a complete shift to IP.

But there remain formidable problems before VoIP calls become as easy as dial-up. First there is the fact that, even on the 21CN network, analogue and VoIP systems will have to coexist and interoperate.

Several companies already offer gateways providing calls from IP phones or PCs to analogue handsets for close to the price of a local call at the country of destination. But there is no easy way to ring an IP phone from a standard handset.

Then there is the problem of addresses. It is possible (if not easy) to dial an IPv4 address, which consists of a 32-bit number, usually written as four groups of up to three decimal digits. Dialling IPv6 numbers is practically impossible: they run to 128 bits, replacing today's acute address shortage with a virtually limitless supply.

The problem is avoided on the web by using the familiar www addresses, which are translated to IP numbers by DNS servers. Zoom/Hayes offers its users a gateway infrastructure called Global Village that gives them email-like addresses of the form user@globalvillage.com that its servers translate into numbers.

If long IPv6 numbers force a general change to this kind of system it will affect the design of handsets, because they will need an easy way of keying in addresses.

It will need a global DNS-like system to allow any pair of VoIP users to link up easily, and to link with gateways that allow two-way interoperability between VoIP and analogue devices.

BT recognises the problem but does not feel that finding the solution is its job. "We just provide the infrastucture," said a spokesman.

It is hard to see how all this is going to affect BT revenues - perhaps even the company itself cannot be sure. But, according to the spokesman: "A simple evaluation of price trends and technology development strongly suggests the recent dramatic downward trends will continue."

It seems likely that online charges will follow the pattern of PC system prices. We pay much the same for a PC as we did a decade ago but get much more for our money; in a similar way the cost of bandwidth will drop but we will use a lot more of it.

So we may still get a phone bill, but it will be for high-res video calls rather than voice.