Go to Settings -> Cellular and scroll down to see how much cellular data each app is using. I would suggest resetting it at the very bottom and monitoring the page. You can turn off cellular data usage for individual apps there as well.
Done that, and will redo it again next weekend when my cycle starts over so I can watch it over the next month.
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Wifi-assist has nothing to do with a massive data increase. You didn't say what phone and what IOS version, but on my phone after about a month, wifi assist uses 8mb. I have VZW.
o Check your icloud settings to see if cellular is being used for backup.
o Do you have a lot of voicemails, somehow visual voicemail uses cellular
o look at the cellular screen and see what apps are using cellular
o when you are on wifi, turn cellular off and see if that helps.
o turn off apps you don't want to use cellular
I'm probably forgetting a few but turning off cellular when you are on wifi should stop the problem although I'm guessing something changed on your phone.
I did actually say, 6S+ running iOS 9.3.5. I don't use iCloud except for Find My Phone. Most months I don't have any voicemails. And at the moment have cellular turned off, but shouldn't have to do that every day when I get to work or when I get home. It's always been smart enough to use wifi when available, cellular when not, and that shouldn't have changed...
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#1. iCloud will only Backup over Wifi
#2. Visual Voicemail does use data to work, but no carrier bills you for that data (it uses a different APN to retrieve it)
#3. Yes... I would go to Settings>Cellular to get an idea of what your top data hogs are.
#4. Turn off "Background App Refresh" on your data hogging apps.
I reset my cellular settings recently, not seeing much informative. I will reset them again next weekend at the beginning of my billing cycle, and watch from that point going forward.
I've turned off most Background App Refresh as well, will see if that makes a difference...
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[doublepost=1473614431][/doublepost]Does anyone think the antenna suggestion could be reasonable? Verizon suggested that a faulty antenna on the phone could be causing it to use cellular when wifi is available. I'm just not sure if that's a reasonable suggestion, or bad attempt at passing the buck...