At the time the original radio series aired, the Islington telephone number belonged to friends of Douglas Adams who did not mind calls, but by 1989 they had moved. If you ring the number now, you will get some people who have nothing to do with Douglas Adams and who are sick of the whole bloody thing. Don’t do it.
(Has anyone made a note of the phone number shown in the movie? Is it the same number, updated for the area code changes? Come to think of it, has anyone tried ringing it lately?)