using mobile to control appliances at home?

[Apologies to denizens of uk.d-i-y who've already seen this, I thought folks on uk.telecom.mobile might have a clue about this too. (I did consider uk.tech.home-automation but there's very little usage on that group.)]
In a Maplin email flyer I've just received:
Remote Control Via Mobile Phone Only: 9.99 Save: 10.00 Code: N34BF Turn equipment on & off from your mobile phone Compatible with most mobile phones No phone charges incurred No need to open or modify your phone or connect with other devices Applications include turning lights on & off, activate a car alarm, control the heating etc.
Any ideas how it works?
Someone on uk.d-i-y suggested it worked by just ringing the home phone a number of times, and the 'mobile' angle was a crimson fish, but that doesn't entirely square with the "Compatible with /most/ mobile phones" bit.
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Velleman used to do a DIY kit for remote switching using a phone. I built one and regularly used it to turn on the central heating on my way home. It worked by picking up the sound of the phone ringing. You had to allow the phone to ring a preset number of times, hang up then redial within a preset time window and repeat. This minimised the risk of false switching by other callers. It sounds fiddly, but wasn't in practice. Another sequence allowed you to switch equipment off again. After a preset period, the system switched off anyway. Because the call is not actually answered, you could control your equipment from anywhere in the world at zero cost! The main limitation - no feedback. However, it worked reliably for me so I didn't feel the need. Don't know about this Maplin gadget, however. I thought maybe it worked using DTMF tones, which would explain the 'most mobiles' bit, but then you would incur a charge.
Blippo
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Whilst at university, a group developed a system like this as part of a final year project. It was controlled by self-built hardware and did indeed work by calling a dedicated exchange (i.e. land line number) at the "house" (in this case it was the electronics lab). As far as I know the landline number DID in fact answer the call, but the calls lasted only a few seconds, hence not much expense spared.
I'd be interested to see how Maplin can implement this for only 9.99. Have you got a link to this?
IIRC there was a Java midlet being sold on the internet not very long ago for the 7650 that allowed you to do this. Again, no idea how it worked.
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I seem to remember talking to an Orange operator once who claimed that the first second of a mobile phone call was free...
Colin
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Colin wrote:

Yes sort of. They allow a margin of error, partly as the exchange clocks may differ by up to 3 seconds. This means Orange only start charging after the third second reported, which may be the first second of the call in reality.
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On Mon, 12 Jan 2004 10:01:34 -0000, "Colin"

This was never true, but voice calls *less than* three seconds long are free on Orange.
--

Iain
the out-of-date hairydog guide to mobile phones
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Colin posted the following for all to hear...

3 seconds.
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Blippo wrote:

I got the email flyer too and looked at the item. It did appear to be a Velleman kit. It seemed to be a pcb, not mounted in a box. Can't find it on their website now ...
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