there's nothing mandatory

It's just common sense

It's bewildering to me how you don't see the contradiction here. If you say it's common sense then it's so ingrained into your work culture that it might not be mandatory but still very well part of the unwritten laws governing how you work together. Not being scolded for not following it does not mean that people will not talk about it behind your back. These rules are the foundation of mobbing and bullying. None of that is an official rule but it can very well be considered mandatory if you say it's common sense to follow it.

it;'s being thought in communication courses etc just like when picking up phone you first say greetings, say company name, then your name.

I was never taught any of this and I also don't do that. Company name first? Never did that and never had anybody mention to me that I should change my behaviour in that regard.

Everything you say sounds weird as fuck to me. Would you mind telling me what country you're from?