I am sure many of us are fedup with receiving calls about boiler grants etc. You may have seen in the paper Mark Hunter's item about a government consultation on the subject. Well here is the link to the Government's web page
https://www.gov.uk/government/consultations/nuisance-calls-consultation
About time they did something. I'm ex-directory, changed my phone number about 12 months ago, don't give it out to anyone unless I really need to and I get 5 or 6 nuisance calls a day. Some of them even knew my name (odd that). However, I bought a new phone with a call display panel. Best thing I ever did. Now, if I get a call from a number I don't recognise, I don't answer it.
A lot of the problem is the predictive dialling system used by many cold-calling companies. To cut a long story short, the predictive dialling system automatically works its way through either a programmed list of numbers (eg British Gas Service customers) or randomly until it makes a connection then dials the caller's agent who can then give the sales pitch or whatever. Sometimes the link fails and it doesn't connect the recipient of the call with the agent which is when you get the silent calls.