Oddly it turns out that these days airplane mode doesn mean turn off all RF transmissions. e.g. it can mean "configure for use in an aircraft under fcc jurisdiction" which means turn off cell radio but keep Wifi on.
Apparently Apple phones will silently phone home an SMS as an iMessage heartbeat when you turned off data.
I bought a SIM card in France, loaded 10 EUR for a 9,95 plan. But my balance declined to 9,85 despite having data turned off and not making any calls/SMSs.
There was no record in Messages, but my provider showed me sending a text.
Ugh. Another 5 EUR added just to buy the 9,95 plan.
> Apparently Apple phones will silently phone home an SMS as an iMessage heartbeat
I recently traveled abroad and bought a local SIM card, and when I first activated it I got a dialog asking if I approved of it sending the iMessage activation SMS. It wasn't silent.
Seems new. I still dunno why Apple made it such a secret.
Drove me bonkers when my carrier claimed I sent an SMS but my phone showed I had not.
Edit: others reported that there’s a message that said “Your Carrier May charge for SMS messages used to activate iMessage” that would still send even if you hit “Cancel”.
Seems like a lot of providers don’t charge for this SMS, but for those that do, it can be a costly int’l SMS.
Source: https://support.apple.com/en-us/HT204234