On Flextel you can set 0871 numbers up to an auto-attendant menu, which means that robot auto-diallers just hang up, but real people can get through. They don'

On Flextel you can set 0871 numbers up to an auto-attendant menu, which means that robot auto-diallers just hang up, but real people can get through. They don't pay as much money though, but some 0871 numbers are free. I think you can also set a divert on anonymous caller (number withheld) You might be able to set two numbers to transfer to each other endlessly, unless the right pair of menu numbers is pressed.