Even with a call blocking service you can only block so many numbers. Some, especially when they are spoofed to legitimate numbers, find their way through.
This happened today with calls spoofed to various 020 xxx numbers. The first just went dead immediately. The second died after a few seconds, but I heard rustling in the background.
The third alslo had the rustling sound......... so I gave a loud blast on the whistle.
Not had another, but I lie in wait.
1471 says the number is 01235874537, Abingdon area, but it is not recognised.