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I was using my iPhone with both data enabled, 3G enabled and inside my hose, where I was connected to wi-fi. I just saw that I exceeded my monthly limit and I was billed $10. So when data is enabled, will it always be used? I understand that 3G is only used for data, so I am not billed for it separately. right?

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You can actually check your used cellular data in Settings > General > Usage > Cellular Usage. If you want to monitor a specific period, you need to manually reset those statistics though. –  Gerry Aug 14 '12 at 14:05
    
It has happened on jailbroken phones and wifi+3G iPads, but these are considered bugs. –  NOTjust -- user4304 Apr 23 '13 at 18:08

5 Answers 5

up vote 2 down vote accepted

If you just saw that you exceeded your data, it could've been many hours ago while you were not at home. AT&T says it could be delayed by 24 to 48 hours.

In my experience, the iPhone will only use data from one connection at a time, not both; Its choice is indicated by which adapter symbol is showing in the menu bar. When you see the Wi-Fi symbol, your iPhone is only using Wi-Fi for the internet.

Example: When I'm connected to a really bad Wi-Fi network (if I am almost out of range or it lost its internet connection), and trying to load a page in Safari, my iPhone will struggle with loading the page for about 10 to 20 seconds. Then, the Wi-Fi symbol switches to the 3G symbol and the page loads using cellular data.

The only exceptions I know of are multimedia messages (MMS) that aren't iMessages and Visual Voicemail – both of these things will not come over Wi-Fi and can't be received with cellular data off. They are not using both, but they could use cellular while you're on Wi-Fi.

Last bit of evidence and future-proofing: In iOS 6 Beta 4, Apple added an option for "Wi-Fi Plus Cellular". (MacRumors: iOS 6 Beta Adds 'Wi-Fi Plus Cellular' Option...)

It will help people who may be on a Wi-Fi network with little or no internet connection be able to get cellular data through to some apps. (Maybe I'm just on the Wi-Fi network for Airplay for example.) It allows you to choose (from their pre-set list) which apps can access cellular data while you're on Wi-Fi. Before iOS 6, this option was unavailable.

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This might not be entirely correct. In iOS 7, a new feature called Multipath TCP Is introduced. (support.apple.com/kb/HT5977) This might be an exception to the above rule. I personally have not seen this behavior though. –  Shane Hsu Jun 29 at 14:00

No. When data is enabled, it will only be used when you are not connected to wifi (iOS was built to prefer wifi).

Yes. Your cellular company (i.e AT&T, Verizon, etc) doesn't know how much data you use over wifi, and they will not bill you for it (obviously, there is some cost for internet in your house, but that is separate). When you are actively using the phone on wifi, iOS 5 will not use any cellular data.

Edit (thanks, Gerry): When the phone is locked, it may turn off wifi (in order to save battery), and use data for some background processes like push mail. Chances are those processes will use negligible amounts of data, but still note worthy.

Be careful of doing large downloads when you aren't connected to wifi. Downloading apps (which can happen automatically in some cases), and streaming videos are two big resource hogs.

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Consider removing the iOS 6 info as this SE does not accept talk about unreleased software. –  Max Ried Aug 14 '12 at 13:08
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Not entirely true. When in stand-by it will turn off wifi to save power, and background processes like push mail will continue to send data over the cellular connection. –  Gerry Aug 14 '12 at 13:10
    
Thanks Gerry. I didn't know that. I edited my post. –  bassplayer7 Aug 14 '12 at 13:40

The phone chooses cellular data over wifi. I was home in a wifi environment on day one of my phone. My phone ran up 17.5 GB choosing data with wifi available and not LTE symbol on phone. I was not connected to the cloud. I was not connected to Siri.

You have to turn off cellular data for every app you install and turn off cellular data period. The reason you do this is because you don't want all your apps using it it the moment you need to turn data on. For example if you use maps. Make sure you turn it off for itunes. Do not stream music. Make sure you do not use the cloud. Keep real versions on your songs on your device.

I had the iPhone 4 since I upgraded to the 6. I never broke 3.5 GB in a month. I broke 17.5 GB in one day syncing my iPhone music library next to my wifi router while physically connected by a wire to my mac. The WiFi symbol was there all the time. That should not happen.

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As far as I know, WhatsApp uses cellular data over WiFi, even if you are connected to both. If you are connected to WiFi only, then WhatsApp will connect using WiFi.

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In the UK an iPhone will definitely use mobile data BEFORE wifi if both are switched on and available, probably a sop to the mobile carrier - remember when these first arrived the MOs didn't even want people connecting via WIFI as they were making a stack of cash with their mobile data.. I learnt this the hard way..

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That's a rather interesting (and worrying) statement. Can you add some pointers to further reference articles or such? –  patrix Apr 23 '13 at 16:54
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No I am in UK, it uses Wifi first, which can be a pain when you pass a shop which has a supplied Wifi as you then have to go and login and it stops using the mobile data you were using –  Mark Apr 23 '13 at 16:55

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