How to Stop Unwanted Telephone Calls

1.) Register with TPS
Under government regulation it is illegal for UK companies to call any individual who has indicated that they don't want the calls.  To do this you need to join the telephone preference service register.  It takes about 28 days after registering to start working.
Website: www.tpsonline.org.uk/tps/
Telephone: 0800 398 893
2.) Randomly Generated Calls
Unfortunately, this is unlikely to stop all phone calls.  Some calls can be the product of random number generators where companies call lots of numbers and hope to get through.  If you are experiencing issues with this the best advice is to record the telephone number of the company down and report them to the regulator - Phone Pay Plus to get the number blocked.
Website: www.phonepayplus.org.uk
3.) Third Party Contacts
Our phone number can also get out into the wider world via whilst registering for websites, doing surveys, filling in competitions etc.  Most companies will have a standard - if you do/don't want your information to be shared with carefully selected third parties please tick/untick this box added to anywhere that you provide your personal details.  The majority ask that you tick if you want to have your details shared but some will have the opposite.  As such out of habit if you only take a cursory glance you can fall foul of this and actually be giving them permission to circulate your information.  It is important to carefully read these every time to ensure that you are keeping your details private.
If you are persistently receiving calls from a particular company it is perfectly acceptable to ask that they remove your contact details from their database. 
4.) Silent Phone Calls
Silent calls are usually generated by automatic calling equipment in call centres.  To reduce the number of silent calls received can register with the Silent Callguard Service, but you need to re-register with them every 12 months.
Website: www.silentguard.co.uk
If calls continue you can complain via Ofcom taking the following steps:
1.) After the call use '1471' to see if a number is provided
2.) If the number is withheld note the time and date and contact your telephone providers 'nuisance call department'
3.) Complain to Ofcom.  Website: http://stakeholders.ofcom.org.uk/tell-us/silent-calls