The database is being built because current Oftel regulations bar BT from using its own billing data to feed other areas of the business.
"We're not allowed to interrogate billing information which would tell us who rings up local ISP numbers for long periods of time, said Damian Peachey, spokesman for BT's consumer division. "This is the sort of information we need to promote BT Broadband."
Because the database is being built up from scratch, Oftel's rules do not apply. BT is mailing potential customers of its broadband service with a consumer survey, which customers can post back or fill in online.
BT is donating £1 to Child Line for every survey completed and has so far donated £500,000.
With a total donation fund of £19m, Peachey says BT hopes to have donated £1m - the equivalent of one million completed surveys - by the end of the year.
The survey includes questions relating to customer satisfaction. "Our chief executive has publicly said he wants to reduce customer dissatisfaction by 25 per cent, said Peachey. "This will find out the drivers behind it to form a top ten list of issues to address. It's a win-win situation for us and our customers."