BT could stop all unsolicited calls from Overseas Call Centres, but, possible, is refraining from doing so until it removes all of its own Call Centres from India.
I sometimes enjoy going along with a Scam Caller. Recently, I had a call from 'Windows' to let me know that my Computer was at risk from attack by a sophisticated virus. In an assumed innocent voice I said which one, because there are at least ten in the house. The Asian sounding chap was a little taken aback and asked me to explain how come there were 10. I explained that it would be even more if Smart Phones were counted because I had four children( total make-believe) who each had a Desktop & a Laptop, and at least two of them had Smart Phones, apart from a PC and Laptop which I had. Furthermore, I explained that my eldest boy worked for Microsoft (another porkie) and had advised me to be very careful about Telephone Scams. Not surprisingly, the caller cut-off.
Hans.