Company surveillance (of their employees) is very real and unlike 'google reads all my emails' they actually can and do and it is a person they actually know whos private information they are viewing. I dont even need to get into the chance of every piece of information being in a lawsuit or get into the lack of any controls around data retention etc to be worried.
First step IT would take when I dropped my phone in 'because they had to run the update' - they would open my photos and take a look through. Second step - they would go to the deleted photos and have a look through. It was done as part of any company-mandated review of the device but out of personal 'curiosity'.
Every private message on teams etc was logged and routinely reviewed by a compliance team and often escalated to line managers - the team doing so knew everything professional and personal going on in the place. Who was getting hired, fired, promoted, working hard, slacking. Who was sleeping with someone, depressed, happy, gay, straight, having kids, getting divorced, getting a nose job, getting a vasectomy etc.
So whats the threat? I have nothing to hide, I am popular, a hard worker, not having an affair with the intern, so they are not going to trawl through looking for any dirt to diminish or fire me. There is little value in this information beyond gossip, but a permanent record remains all the same. For me there is little extra effort required to phone home from my phone rather than the recorded office line and that way the call wont be listened to by the guy in compliance.