I recently read about a study that tracked how much iOS and Android phones phone home. The abstract says it all (emphasis mine):
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, handset phone 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. Users have no opt out from this and currently there are few, if any, realistic options for preventing this data sharing.
I was inspired to write a post about this topic (and the OpenSnitch project), which you can check out in the link below.