My friend with an iPhone 4S found he had reached his data limit (512mb/month) in just one week when he picked up the new 4S. Mind you he was probably using it more to play around with at first, but he does use it for email and texting a lot. He upgraded to a 6 gig plan.
I was surprised how much data it uses. I guess its using data for any question you have, even if its about content on your iPhone. Anyone else notice this? My use if Siri has all been on wifi.
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SIri doesn't really grab much data - she returns text answers. But the uploading of the audio to Apple servers is where you could rack up lots of data.
If it's mostly downloaded data, then I would blame it on an app-happy new iPhone owner.
Location: Markham - Soon to be Mississauga, Ontario
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Tell him to go through the settings, one would be the automatic downloads in Settings->>Store another is iCloud, both have a settings to use Celluar Data.
Basically, if you make 11 Siri queries a day every day for a month, you're going to use about 20 MB of data in total for the month.. Not much at all... I think your friend has used up their data doing something other than asking Siri questions..