The telecoms giant is not setting up a new network, but will purchase network capacity from T-Mobile.
The move recognises that younger people prefer mobiles to landlines – and BT needs to protect itself against a decrease in the value of the landline market. So BT's new package will link both home landlines and mobiles into one contract.
Pierre Danon, Chief Executive of BT Retail, explained:
"We believe BT is the first company to recognise that customers are not just individuals, but parts of families. Family members who are likely to already have an existing relationship with BT can now also get all their mobiles from us on the same contract, taking the hassle out of managing different call charges and line rentals. This is a perfect solution for any modern family."
According to BT the package will cost £15 a month for line rental for the first mobile, and £10 per month for up to five additional handsets. A single bill will cover all the family's mobile use, and a usage alert service will keep parents informed on how much is being spent and by whom.
Customers will be offered free calls to their home phone number - if the calls are less than two minutes long. According to BT research, people phone home on average five times a week and most of these calls are for two minutes or less.
BT hopes to have the new package available by the end of October.
Other players in the market were sceptical about the success of the venture. A spokesman for mmO2 told BBC Online that it was a "bad idea, psychologically" to use one family bill, because the charge would be seem greater than if separate bills were used.
Vodafone's chief executive, Christopher Gent, told the Independent newspaper that "With regard to BT, you'd have thought they'd learnt their lesson from their last attempt at doing mobile - but never mind".
BT also announced that is also running a trial using Sony Ericsson bluetooth-enabled phones to route mobile calls over the fixed network rather than the GSM mobile network, lowering customer call costs and improving sound quality.