The Moral Dilema

I told a lie at work today. It was (yet another) sales call, the ones where someone rings up trying to sell computer services/water coolers/ goodness knows what else. They ask to speak to the office manager, but if I try to put them through she just says "no thanks" and leaves me to get rid of them.
So I say that she's not available. Unfortunatly, this means they keep ringing back because they think they might get through another time. But when I know that she's not going to take the call, and doesn't want to be disturbed, I will still say that she's not available. Which isn't quite true. She is there, she just doesn't want to talk to that caller (and I don't blame her).
But I can't tell the callers to stop calling or that we're not interested (although I do try to sometimes) because I'm only a receptionist. One caller I tried to get rid of got quite angry. So the same people try calling over and over again. Some just don't get the hint, even after weeks of being told she's not available. So it's not a very good system really.
It's the same when sometimes the secretaries will say that they don't want to speak to someone because they're too busy; "can you tell them I'm not here?" I don't like lying, so the great phrase "not available" is very useful. It can mean "they're in a meeting" or "they're not here" or whatever the caller wants it to mean. Including "they're here but they're refusing to speak to you." It generally does the job, unless the caller starts asking where they are.
I don't like not telling the truth. I don't like being put in the position where I have to lie or get into trouble for not doing my job properly. And it would be much easier (if less polite) to just tell the sales callers to stop ringing up. But it won't happen. I guess it just shows what a mess the world's in!