Thousands of nuisance phone calls that plagued people in west London this month have been traced - to BT.

The company admitted responsibility for the calls, in which subscribers answering phones were greeted with silence.

It started on 13 August at the Perivale exchange, when a code that should have diverted computer-generated check calls back to the exchange was miskeyed - and residents spent a weekend answering dud calls at all hours.

John Lawrence, chairman of the Buckingham Avenue Tenants and Residents Association, had at least 50 people contact him. 'It happened to many people. The most annoying thing was it was occurring during night hours. We had one lady whose phone rang seven or eight times from nine in the evening until six the following morning.

By the following Monday morning, BT had received 150 complaints. A spokeswoman said 20,000 people on the 991, 997 and 998 exchange could have been affected. The calls were part of preparations for transferring those exchanges to new digital systems.