To stop junk mail, you can register for free with the Mailing Preference Service. This means that you will not be contacted by any business you have not dealt with previously.
To stop getting unrequested sales or marketing telephone calls you can register for freewith the Telephone Preference Service (TPS).
On the above website your attention is drawn, specifically, to:
Stopping silent calls
Silent calls are generated from the use of 'automated diallers'. When these are incorrectly set, more numbers are dialled than there are operators available to take the calls. If the purpose of the call was to sell you a product or service, then registering on TPS will reduce these.
However companies use automated diallers for things such as debt collection, market research etc. and TPS will not reduce these. Please see the Guidance Notes which the Information Commissioner's Officer have produced - Privacy and Electronic Communications (EC Directive) Regulations 2003 and the Ofcom website.
If you want to reduce these types of calls there is a separate service called Silent CallGard. You can register with them by calling 0844 3722325.