The problem is that there can be a fine line from a normal thing I'm happy to help with to people taking liberties that are causing problems, and it's not always systemic. As I said in a different comment, I have been fortunate that most jobs I've had have not abused their access to my personal contact information, which is why my colleagues tend to have mine.
However, I have had the problem with things like email. For example, when I wrote and maintained a piece of internal software, people could email me with support requests and I would be happy to answer them. Most users did this rarely, and in that case, I didn't object much even if their question could be solved without my help with the documentation we already had. A small number of users, however, decided that I should be emailed every time they had any question, no matter how minor, and that if I didn't respond quickly, they should just send more emails. That kind of thing can cause several problems. It wasn't my manager or my team. It wasn't even very many of the users. It was just a few people causing problems for everyone else. The risk with answering the odd call is that you suddenly get more odd calls.