Nuisance phone calls.

I am not on the whole a rude woman. I listen usually with great patience to people explaining their difficulties  most days of my life but recently I have started to respond to certain calls in a fairly unsympathetically  way.
The phone rings. I pick it up to find no one on the either end. I say hello. Nothing. I put the phone down.
That scenario may happen several times in a day. Sometimes I hear the sound in the back ground of a busy office. Other voices speaking , phones ringing.
Sometimes the phone goes dead the moment I answer.    If I was a suspicious woman I'd assume it to be "The other woman"
Then sometimes someone speaks to me. They say. "Hello Mrs .... how are you today?"
This simple question makes me very cross now. Its obviously being used as a way to get people to respond...I have often wondered if you started to tell them how you really are it might put them off a bit.
Next time I could tell them about a bad back or an anxiety about a trivial problem.  I could stretch it out a bit  to see what happens.
They don't really want to know how you are though. Its just to get the ball rolling so the ball goes no further with me. I pick it up and lob it back!
This morning I said in answer to the question....."Look just tell me what you want please."
The young man with poor English asked me to repeat the question at the end of which he went into his spiel about getting me 40 % off my electricity bill for the rest of my life......which might not be long at this rate.
Why do they do it? Who thinks its cost effective to have a call centre in India?
The real  problem here is that I have twice now responded badly to a genuine question as to my well being....  then its difficult to explain!  If the number says International its usually a sales call. If its says the number is with held..same thing  but just occasionally its a call you really need.....this whole selling by phone thing is an actual nuisance.....how do we get it stopped?