There's been a lot of publicity for 13-year old Laurence Rook of Whyteleafe, Surrey and his invention of a doorbell which automatically calls the householder if nobody answers the door.
The idea presumably is that if someone calls with a package the occupant, if out, can ask that the package can be left with a neighbour, or rearrange the delivery. Sound is added to give the impression that an intercom is being used -- though the person doing the delivery may be baffled why the householder doesn't simply open the door.
UK News Gateway has a page which states that he will make £250,000 and that orders have been placed, and that the invention has been "patented as the Smart Bell technology". Other articles credit family friend Paula Ward, also an inventor, with helping with the design and with getting prototypes made.
Full credit to Laurence for thinking of the idea, but you cannot use a technical idea if it is already covered by granted patents, and I wonder if a search was carried out. Despite the statement in the article, I cannot find any published patent documents in the name of either Rook or Ward. Nor do you patent a product in a certain name, which is the realm of trade marks.
After checking the Searchable Patents Journal, which includes records of newly filed British patent applications, I did find on the Patents Status Enquiry site two applications made by Paula Ward on the 9 December 2008, GB0822379, "Always answer the door", and GB0822380, "See who's at your door", both of which terminated before publication. There were none in the name of Laurence Rook.
There are in fact quite a few patent documents out there for the idea of a doorbell activating a call to a telephone. The idea is graphically represented by the illustration given below, which is from Denis Hunter's British patent application Entry communication system pre-programmed to call a remote landline/mobile telephone. It was published in 2003.
It didn't get patented, as there was "prior art" -- anticipations of the concept as shown in earlier patent documents -- as listed on the final page.
The many patents on the subject are rather scattered, but the prior art list for Ben Cornelius' British patent application Calling a remote telephone by doorbell activation and controlling entrance to a building from the remote telephone, published in 2007, and also not granted, is a useful starting point.
Prior art often invalidates the patenting of excellent ideas by showing that they are not in fact new. Sometimes they also show that a still current patent (they can last for 20 years) exists for the idea in a country where it is planned to sell or make the product. That means that besides not gaining a monopoly in the idea, which investors always want, you would be infringing a patent.
Generally speaking, it is not the idea itself but the precise solution which is patented, so different companies can patent different solutions to the same problem. A prior art search can also reveal a range of interesting solutions if only the problem, and not a possible solution, has been considered. Advice from a patent attorney should always be taken to decide on the course of action.
Prior art searches are hard to do on the Web without help, and I suggest asking for help at a patent library such as in Europe or in the USA, or else the local patent office.