Many people I know who have answering machines or voicemail (myself included) have them set up to answer fairly quickly after it starts ringing. This means that often, even when the person is actually there, they don’t manage to answer the phone before the recording answers it automatically. Often, though, I don’t want to leave a message, either because it’s not important enough, or because I need to ask them a question and there’s no point asking a machine.
My solution to this is to call them twice. I call the first time, wait a few seconds, then hang up. This gives them time to get to the phone. After a brief wait, I call again. If they’re available, they’ll answer immediately, because they’re alert and right next to the phone. If it rings for more than a few seconds, then it’s almost certainly going to voicemail, so I hang up. I can tell by how long it takes to answer whether I’ll be talking to a human or a machine.
I call this the Two-ring Test.