Vodafone customers seem to have been mostly affected by this scam - at least, so far.
What happens is that the scammers set up an 0845 number so that calls to it are charged at a high rate. Then, they hack a user's mobile phone so that it calls back to a missed call from the number. The number calls the mobile and disconnects immediately, and the mobile duly calls back.
The user doesn't know anything about the inbound call, or the return call, which usually happens at night.The call can be connected for up to twelve hours, and eventually the user receives a hefty bill from the phone company. The call was made from the user's number, and as far as the phone company is concerned, the customer made the call - people were being forced to pay up.
The phone companies are obliged to hand over a percentage of the call charges to the people who operate the 0845 number in the normal way.