So, I am not a geek but there are apps who keep working in the background and it seems that even when you turn the roaming off it is not enough to stop them. So as suggested above the only way is to turn mobile data off.
As long as you use an Apple device in the way Apple wants you to use it then you'll generally come across very few showstoppers, and if there are then they'll generally get fixed very quickly.
It's when you want to use an Apple product that's outside of Apple's "way to do things" that you'll come across a myriad of bugs - most of which either take an eternity to fix or may never be fixed, despite the options to do those things being present.
I always have roaming off and have incurred these charges once before as well--Orange (at the time) cancelled the cost on the bill when I called and complained. However, since then, I have also switched off my mobile data before I reach any border and have had no further issues (on my android phone). It was so long ago that I can't even recall if I had switched off my mobile data on that first trip or not, but I'm guessing I forgot to or didn't realize such extra charges could occur.
I had a similar experience just this week. My phone according to Salt, racked up between 200 and 500mb within seconds of me leaving my home for several days. thus using up my included data bundle. I contacted them and had to buy an addtional bundle to keep me going until my next billing period and lo and behold my data usage is back to normal levels.
As an aside I went InterD to get a phone for my kid today. When I said that I wanted the Coop contract, he said wait 2-3 weeks until Salt go out of business. Obviously I laughed... but he didn't, seems Coop are looking to move their service off the Salt network.
I've just received the bill from SALT and I've realized that salt charges voicemail roaming twice, even if your phone is switch off and you SIM card is out: first they charge voicemail as incoming call, secondary as outgoing. Be aware! Compare to my prevoios mobile provide they are sinking in S**T. I'm not going to stay peace and quiet - I'm finding that terribly mad and absolutely nonsense and I'm sure I'm not alone.
Salt advertising campaign doesn't reflect the reality - they are too far away in many levels.
I had the same problem with my iPhone for a couple of months. I contacted the customer service and they seemed to be aware of the issue. Here is what they wrote back:
"You probably have auto updates switched on your iPhone 6 and that trumps the fact the the data connection is switched off.
I can say this with absolute certainty regarding the data volume (30KB) you used up. On the iPhone, when the Wi-Fi Assist detects a weak Wi-Fi signal, automatically switches a user’s iPhone over to a stronger cellular signal, if available, so that you don’t experience any of the symptoms associated with poor Wi-Fi, such as drop-outs and buffering."
Switching Wi-Fi assist off ( Settings -> Mobile data -> (Scroll all the way down) Wi-Fi Assist) worked for me and I got the amounts refunded by Salt. Hope this helps!
curious thing with our salt subscriptions, our latest bill shows a load of texts sent to abroad, something like 25 for me and 20 for my wife, we checked and we only sent about 5 each, very odd
This is not unique to Salt, all providers in Switzerland charge for voicemail when you are abroad. It's called Voicemail Tromboning, it's an unfair practice of course but there is nothing you can do about it other than disable your voicemail before you travel.
And if you receive an SMS abroad, even if roaming is off, this counts against your free roaming days or you get charged for roaming. Very annoying, now I need to see how I can switch that off
I'm using M-Budget (Swisscom) and before that I used Swisscom itself for over 5 years. I've never been charged for people leaving me a voicemail if my phone has been off at the time. As far as I understand it, you only get charged this double-roaming fee if the call actually comes to your phone and then gets sent (back) to voicemail. If the phone is off, or in airplane mode or whatever, it tells the mobile network of the country you're in that it's off; this quickly gets reflected on the Swiss network and voicemails get taken without the call leaving the country. Certainly that's how it's always worked for me here. So far.
That sounds weird, as receiving a text is free and not part of any roaming agreement.