diy caller id and deal with
Even though its too late I really had to put a comment in here. A few years ago I lost my rag with the constant worrying from double glazing companies, particularly Zenith windows, a name you will well remember. After deliberately wasting half a day of a salesmans time, in retaliation, like, and receiving various threats after attempting to do exactly the same thing again when they next called, I built a device around a PIC controller which displayed the incoming number and stored it , with the option to assign various actions to the number. These options included: open line, delay enough for the (money) charging relays to operate then hang up before the first ring, disconnect ring line (let it ring) , and chop up the ring cadence into distinctive patterns to assign to special numbers.
The device was battery powered, such that there would be no fear whatever of feeding dangerous voltages back into the phone lines, and AC coupled to the lines, such that no extra loading could be detected.
To date, the device has successfully eliminated calls from WITHHELD, UNAVAILABLE, INTERNATIONAL, anything starting 08, including BT, and Birmingham.