It earned the dubious honour of being the first TV programme to be cited in the British Medical Journal as having caused Post Traumatic Stress Disorder in children.
The rumour that the show's makers had wanted to include a high-pitched tone that would agitate viewers' pets during the broadcast is mostly true. Writer Stephen Volk suggested it, but as soundtracks are 'capped' for transmission (meaning, limited in pitch, volume, etc.), even if permission had been secured to go ahead with the idea, technically-speaking, it would not have been possible.
While you may not see him 'Pipes' appears many times. To name a few: in the bedroom after the 'face-cutting' scene, behind the door to the cellar in the closing minutes, in the glass as 'Sarah Greene' picks up the posters, behind Parkinson at the end and even in the crowd at the Park as Craig Charles walks around - there are also many others.
Now obsolete, the 081 811 8181 phone number was real (and the standard one for genuine BBC shows); the first thing people were told when they rang up was "this is not real" although they were still allowed to tell their stories. However, there were only five operators receiving calls, so some viewers trying to get through only received the 'engaged' tone, indirectly helping to spread confusion.
Ghostwatch was never supposed to trick or deceive the viewers. It was billed as a drama and contained a "written by" title card at the start. The majority of people tuned in late to the programme after a film finished on ITV and therefore many thought what they were seeing was in fact going out live.