The system, now on trial in Glasgow, would let several people in the same house each have an "assigned" ringing tone - for example the normal two short rings or two long rings. They would then know precisely who the phone was ringing for.
BT has had the system on trial since last July among a few hundred subscribers. The company says that its introduction nationwide is "a matter of when rather than if".
The move is part of an effort to fight back against cable companies, which are using their own recently installed digital TV and telephone networks to offer a wide range of services.
Cambridge Cable, which owns four franchises covering a total of 500,000 homes, has been offering the same "multiple ring" service - IdentiCall - since December at pounds 6 per quarter per extra number assigned to the line. Other cable companies are also planning to introduce the service.
The system uses a facility available through digital telephone exchanges by which a particular phone number does not have to be connected to an actual line. Alternatively, many numbers can be assigned to a single line, with a particular ringing pattern, controlled by the exchange nearest to the phone.
It could be used by somebody working at home who wanted to distinguish between incoming business and personal calls (which would be listed under different numbers in a phonebook) or by families who might want to offer teenagers a separate phone number.
Outgoing calls would still be charged to a single bill, but itemised billing would make it possible to identify who made which calls.
"While the technical limit is 16 lines per phone, it gets increasingly difficult to distinguish who is being called as you add more," said a BT spokesman yesterday. "It's easiest with two, and then it gets progressively more complex." The problem is that the exchange can vary the length of individual rings, but not their pitch.
Multiple-number facilities have been available for some years in the US, where digital exchanges have been in use for longer.
BT only converted its national network from older, analogue systems in the middle of last year.Reuse content