BT doesn't "allow it", Grammaretto, because no-one asked their permission. It is being done by the same scammers who phone you to say that they are your bank, or that they have heard that you had a car accident, or that thay are from Microsoft to tell you that your computer has been hacked. They are phoning from a thousand different phone numbers, changing as soon as they think they might have been traced. BT and all the other firms whose names have been taken in vain, and a lot of scam-detection firms, are trying hard to keep up with the villains. You can help by informing BT and any other organisation of these scam calls.