I assume a jailbreak would soon sort that. I don't know how they would detect it either.
The tethered device requesting data reports itself to their network, or the network requests it to report itself. From that information they know "...whether the device is a mobile phone or not". I can't remember exactly how it's done - it has been *years* since I had anything to do with mobile tech, but do know for certain that is the way GiffGaff know whether you are breaking the T&C's.
I've been using my phone as a wifi point for my passengers all day and had no problems with it so I assume it's all good.
You're not a bus driver, are you? That'd be a cool thing for a bus driver to do...
Edited by deleted (Tue 25-Mar-14 06:53:05)