Had this a few days ago...
Was an American accent gentleman stating he's calling from BT about my broadband connection - they detected 25 simultaneous connections all going through my internet line the evening before, and that I had been hacked! He was about to cut off our internet service for "protection". I was immediately suspicious. He was persistent, giving me a "BT case number", his "name" plus other information connected to BT (all fictitious). He then asked how many electrical devices in my house use the internet, plus to name them.... At which point I hung up.
Doing more research, a similar type of call is automated, about an issue and to speak to one of their advisors press " 1 " to be put through to their service desk. It then however dials out from your line to a premium charged number (that you don't realise is happening until you get your bill).
Researching afterwards, the above scams seemed common knowledge to some, however I thought I'd post this up as a caution.