Indian call centre scammers are targeting BT customers
http://www.theregister.co.uk/2017/09/06/bt_call_scam/
Be wary of any unsolicited calls and hang up if there is any doubt that you are not speaking to the real company. Do not give any personal information away or confirm any details they tell you. Genuine call centre staff do not get cross or aggressive and this is another sign to look for if you are unsure if the call is genuine and you do not confirm or give personal information.
If you have an Android or iOS Smartphone try using TP Protect https://tpsprotect.org.uk/ on your mobile to help rate and filter out unwanted calls.