iPhone cellular data has zoomed up the last two months - I wonder why

My iPhone cellular data is typically well under 3 GB a month. Mostly because I’m usually at home and using wi-fi.

But the last two months my cellular data has exceeded 7 GB a month.

Upon first check, it appears that “system services” (mostly syncing Desktop and Documents) was using most of this data. So I went into Settings > Cellular and turned off syncing over cellular data.

That slowed things down, but I still wonder - why? It’s never happened until recently. And why would cellular data be used for that all of a sudden since my iPhone is on wi-fi most of the time?

There is some cost impact to this. About 6 months ago I switched to the new Softbank “Merihari” plan, which gives me unlimited 4G and 5G data with no slowdown at a lower cost than my previous 5 GB plan. But that plan also includes an extra 1,500 yen/month discount if my data is below 3 GB/month. The first few months that was the case and my month bill dropped from about 10,800 yen/month to 7,500 yen/month.

But last month and this month I didn’t get that discount, so my bill was about 9,000 yen. Still a savings over it was before, but a waste of 3,000 yen for some unknown reason.

Any idea why my iPhone could be using so much cellular data?

I don’t know why, but check Settings / Cellular, scroll to the bottom, and see if WiFi Assist is turned on. If it is, then your iPhone could use cellular data rather than WiFi if it is getting a poor signal from WiFi. (I usually keep that option turned off, but I believe it’s on by default.)

Wi-Fi assist is turned on but this period (there are 4 only days left in this period) it’s only used 48.8 KB it says.

I haven’t noticed an increase in mobile data use recently (but I have held off updating to iOS 14.6j. I am mainly connected to a home wifi - especially during a covid lockdown!
For reference I checked my use of mobile (cellular in the USA) data:
Settings/Mobile/[scroll way down to the Mobile Data heading]
Safari is my biggest app user at 2.8Gb over many months
Personal Hotspot shows “other devices” at 3.2Gb. I suspect this is mostly the sat-nav in my car.

Wi-Fi assist has only used 163 KB so far this month, but cellular data continues to grow. Documents & Sync is still growing by several hundred MB a day. At this rate I’ll lose my automatic 1,500 yen discount again this month for staying under 3 GB.

I talked to Apple Support, but the advisor I spoke with was completely unhelpful.

I am on an inexpensive pay-as-you-use cellular plan on Ting (about $30/month for two phones). I keep everthing turned off on “Cellular Data” except for “Phone”. (I discovered that “Phone” needs to be turned on for Text.)
Everything else, especially iCloud and Sync, can wait until I am on WiFi.

That’s what’s weird. In cellular settings I did turn off iCloud Drive. So why is the Syncing and Documents using so much cellular data?

It could be the WiFi Assist. I read somewhere else that it could induce flaky behavior, so I made sure to keep it off. Maybe it allows other services to run over cellular but doesn’t count their data?
Try turning off anything else that is “On” for cellular (if anything else is “On”) and see what happens. Good luck!

Wi-Fi assist is only using 163 KB this month so far.

It would be great to find a solution that didn’t require manually turning off and on cellular data when needed. Aren’t the value by each app correct as to data used?


I don’t know what the rules are for counting data, so “correct” could be debatable. Does the app count everything it enables? Does it count only its own discovery protocols? There could be complex conditions on what counts.

I have the same problem on my iPad mini. I’m grandfathered on ting, and the first data bin is 100 MB. Since I’m almost always home now, I rarely use more than about 70 MB. But Apple System Services use 300+ MB, costing me an extra $10 per month. I prefer cell data instead of wifi to stifle any app that doesn’t explicit require to connect to the net, and I only turn on wifi for things like app updates which I have to schedule carefully anyway due to very slow wifi. Background App refresh is turned completely off. Low data mode is on for both cell and wifi. Notifications are turned off for everything but Find My iThing. 90+% of my iPad use is for reading epubs in mapleread which isn’t allowed to use cell data.

last billing period the top Services usage:

MB Service

72 Security ??
42 Apple ID
30 General ??
27 DNS
26 Documents & Sync I have this turned off everywhere possible on all devices
22 iTunes Media services I haven’t used any apple media for months
8 iCloud the only thing I use iCloud for is Find My iThing
4 Apple Advertising I have zero ad-supported apps

then a ton of little stuff

The iPhone is similar, 300-500 MB per month for Services. Less of an issue since I have 5GB, but still annoying.

While too late for this month (I legitimately used more than 3 GB this past month while out and about as an Olympics volunteer) I do think I stopped the “system services” from growing. I look forward to next month.

My theory is that somehow syncing Desktop and Documents over cellular data got turned on. Then after I turned it off there was just a delay in the reporting of the data. Because after a time its growth basically came to a halt.

I’ll let you know what happens next month.

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