

It's also rumoured that some phones have a second set of APN details saved in them by the phone network, when you enable tethering they switch over to using this second APN for all tethered traffic, while using the normal APN for traffic originating on the phone.


Your phone tells your network that you are tethering This generally only happens if you are running an OS version that has been customized by your Provider, example 1 example 2.
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The first and easiest method is that some phones will query the network to check whether the current contract allows tethering, and then totally disable the tethering options on the device in software if not. However there are certain known techniques that will give away the fact that you're currently tethering, if your Service Provider happens to be running the right tool to check for these indicators: Your Phone asks your network if tethering is allowed How they detect that someone is tethering a device isn't something that network providers often want to talk about, for the obvious reason that the more consumers know about how this is being detected, the easier it is for them to find ways to hide the fact that they're doing it, and avoid the associated extra charges (1).
