You could always sign up for caller display, which is free on BT (TPS can be done at the same time) and then subject to the right phone, you can check the number before answering it. I now know that any calls that have "number not available", "International out of area" or the such like can be ignored (I just switch the phone off) or I speak to them in my own made up East Europen language from the second I pick up the phone .

EDIT: This is BT privacy that I signed up for, as mentioned above, and you get TPS and Caller Display. The only problem is if the bank has to call you back and you have a bank that uses a foreign call centre - although it is the fault of the bank for using a foreign call centre.