I missed this yesterday...
A quote from a letter the Professor wrote to Joy Hill at Allen & Unwin (his publishers), on May 10, 1966... She apparently had recommended that he get an unlisted phone number....
"Thank you very much for your suggestion about my telephone number, which I will consider. Removing the number from the directory seems better than the method adopted by Major W.H. Lewis to protect his brother, which was to lift the receiver and say 'Oxford Sewage Disposal Unit' and go on repeating it until they went away. "