Sometimes you may try to phone someone (whether that's me, or anybody else, in general), but never get an answer. There could be any number of reasons for that, but here's a few:
You didn't let the phone ring for long enough. We're not always sitting a desk with the phone right by our side; we might be in the middle of doing something, and we might have to make our way over to where the phone is. Let the phone ring for a lot more than just four seconds.
We are out, and the answering service has failed. Try calling again, later, or try calling the mobile phone number.
There could be phone company service fault. Yes, you can have ringing tone in the earpiece at your end, but with nothing ringing at the other end. Try calling again, later, or try calling our other phone number.
We just weren't anywhere near the phone, and never heard it. Try calling again, later, or try calling our other phone number.
We are busy with something, and can't interrupt it to take a phone call. Try calling again, later. On this note, if we're in the middle of doing a job for you, you're much more likely to have us interrupt it and answer the call, if we can see your caller ID and recognise who's trying to phone us.
We are in the middle of another phone call, perhaps a very long one, and just can't break away from it to answer another call in the middle of it. Try calling again, later.
If your call gets answered by a machine, don't hang up immediately. It could be that we're on another call, or that the machine grabbed your call moments before we picked up the phone. And if we don't answer, leaving a message for us to call you back is a good idea.
We've had a series of annoying nuisance phone calls, and are holding off from answering calls from unknown numbers for a while. Try calling again, later. If you stop hiding your caller ID, you may find that you don't encounter this problem, any more, with a variety of different people that you can't reach on the phone. Even if you use caller ID blocking by default, you can turn your caller ID back on for one phone call, by dialling a prefix code before the number (for Telstra users, you'd dial in the numbers 1832, followed by the phone number that you were calling).
Perhaps your caller ID is one that we've chosen to ignore on purpose. Hiding your ID to get your call answered will probably not work (see above note about unknown numbers), and we have no qualms about hanging up on nuisance callers mid-sentence.