On a German security blog I just came across an interesting study (just published last week) showing that both, iOS and Google Android, transmit telemetry, despite the user explicitly opting out of this.
We investigate what data iOS on an iPhone shares with Apple and what data Google Android on a Pixel phone shares with Google. We find that even when minimally configured and the handset is idle both iOS and Google Android share data with Apple/Google on average every 4.5 mins. The phone IMEI, hardware serial number, SIM serial number and IMSI, handsetphone number etc are shared with Apple and Google. Both iOS and Google Android transmit telemetry, despite the user explicitly opting out of this. When a SIM is inserted both iOS and Google Android send details to Apple/Google. iOS sends the MAC addresses of nearby devices, e.g. other handsets and the home gateway, to Apple together with their GPS location. Currently there are few, if any, realistic options for preventing this data sharing.
The info on this study I found in a blog post on kuketz-blog.de, really an interesting source of information for all those interested in privacy matters and IT (in German).