? Do you always have to call?

I'm new here, but where I live, we generally don't answer the door, unless we're expecting someone. This is probably due to hearing about home invasions ("push ins").

If it's immediate and close

I prefer a phone call before someone comes over, that way I can ensure that I will be home to visit.

I think it's safest to call people before you visit if at all possible. Most people seem to expect it. But I grew up with people just dropping by and doing the same, so I generally don't mind if people show up unexpected. Like others have said, people are coming to see me, if they are offended by the state of my house, then they learned their lesson to give me some warning first! :D I know most people want to be called first, so I do that.

If people show up unexpected, I usually invite them in for a few minutes at least. I don't drop what I'm doing...I either cut the conversation short and send them on their way or continue doing whatever chore I was busy with while we chat. If they are casual enough to drop by then they shouldn't mind if I'm casual about them being there. :)