The machine, in the office of the MoD Secretariat (Air Staff), is the one conduit where the public can pass on sightings of UFOs to the Government.
Even believers admit that the vast majority of UFOs reported turn out to be IFOs - identifiable flying objects. But for some people there remain sightings that cannot be explained by reference only to the terrestrial. In 1997 the machine took 250 calls. There have been 56 so far this year.
"We check the machine every hour," said an MoD spokesman. "It is easier to have the answer machine because otherwise we would find ourselves having all sorts of people who claim they have seen all sorts of things."
The position of the MoD is that it is only interested in UFOs if they constitute a threat tonational safety, as the answering machine message makes clear.
But most of the reports are made hours, days or even weeks after the alleged sightings, by which time any threat would be long gone.
In any case, the MoD remains confident that any genuine threat would be picked up by its huge pyramid radar at RAF Fylingdales in North Yorkshire, which constantly monitors the skies.
UFO interest groups say the number of sightings reported to them is steadily rising. Between September 1996 and August 1997, the British UFO Research Association (Bufora) received reports of 386 sightings.
Glenda Dixon, the association's director of investigations, said that up to 95 per cent of sightings could be explained. She said that most sightings were actually ordinary aircraft, laser lights, or even bright planets viewed through binoculars. Satellites, weather balloons, birds, kites and reflections also accounted for some false alarms. Bufora also believes that many sightings are of test flights of new, secret, military aircraft.
The number of reports of "close encounter experiences" is also increasing, Mrs Dixon said. "The close encounter experience is a problem, as although it is interpreted as being like a UFO encounter this may not be the case. These extraordinary events could be linked to other anomalies such as paranormal events which may or may not include encounters with UFOs and aliens."
One reason for the increase in reports is the huge amount of coverage that UFO and associated matters now receive in the media and on television programmes such as The X Files. The opening programme of a new BBC television series of Everyman on 24 May will include the testimonies of people who claim to have been abducted.
Malcolm Robinson of Strange Phenomenon Investigations, which has offices in London and Scotland, said: "People are becoming more prepared to report their sightings."
He agreed that around 95 per cent of all reported sightings can be explained. It is the remaining five per cent that interest him.
"I got into all of this in an effort to disprove it," he said, "but you realise that, once you have swept away all of the debris, there is a hard core of cases which cannot be simply explained."
He recalls the case of two men from Edinburgh who claimed to have been abducted by aliens as they drove home one evening. "They had no interest in UFOs at all," he said. "I spoke to them at length. They are genuine people. They believe they had a close encounter and I believe them as well."
Anyone else believing they have had a close encounter, or wishing to report a sighting, can leave a message with the MoD on 0171-218 2140.Reuse content