Hi Jez,
Some of my reply may be not what you want to hear, but based on being best prepared for battle I'm playing the game from both camps in order for you to prepare. I'm not legally trained but have a stubborn and vert cunning streak when arguing
I have had mobiles since he early satchels made by panasonic, back then the cost of the calls was very high, pay as you go was not available. when data services started to roll out they were all charges on data received or and sent.
It is new to me to be charged usage by time, had i been on you provider i too might have been caught out. So don't feel embarresed. You need to see if this is the norm in your country, and is it mentioned in you t & c's.
My iphone4 (fairly new) has data roaming switched off, and I'm sure that's the factory default. This works for you as if Apple warn by turning it on are aware of high costs so even have a message to warn you, so must the provider be well aware. If they have remotely switched it on without pre-warning you, I'd think a good case can be made against the provider/s.
A bad thought is other types of services set a prescekdent for chariging by time, like taxi's. You get charged for distance but also for time while stuck in traffic, so be careful with choosing your words when making comparisons
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If they don't want to compromise then swallow pride and go seriously public, emphasis your young son unaware of doing anything wrong stayed on line to download games and content. Why was nothing done? if the provider could warn you at $15000.00 AUS, they coukd warn you by text @ $1000.00?
They can't claim they don' t have the systems in place, it's a matter of threshold settings being set and it's th actual owners of the networks who's computers calculate this.
You could really paint them in a bad light IMHO.