Hi Freebiker,
Interesting use of mobile phones! Just a couple of points that are wirth mentioning -
GSM Jammers will jam the internet connection and the normal network signal nd work with a lot more range than 20 metres.
Hooking the phone up to a relay to switch the engine off is illegal, proceed with caution.
As with everything else you pays yer money and takes yer chance. I am biased, of course, but I still think that sometimes it is worth paying a little bit extra to know that the device will work whenever you need it to, not conk out because it's atired old phone that hasn't been designed to operate in the way it is being used.