If someone calls you or you need to contact someone within the UK, you can employ several methods to track her phone number. With access to the Internet and by using your own telephone, you can easily find a number or discover who owns a given phone number.
Make sure you have the entire phone number, including the three-digit area code along with the country code for the UK, which is +44. Trace the origins of the number by searching for the area code, which will tell you from which part of the country the number originated.
Visit the online white pages for BT (British Telecom). Though you will not have the ability to perform a reverse telephone searches, but if you have a partial name and location you should be able to do a wild card search.
If you live within the UK, call +118 500 for directory inquiries. You will need to pay a small fee for this service depending on whether you've called from a landline or from your mobile, and whether you've called from another network.
Visit a free online mobile phone directory such as Mobile118, which offers free registration and allows you to search via name or location.
If all else fails, hire an investigator or use an online service to trace a landline or a mobile number. You'll generally need to pay a one-time fee for this service, depending on the details you wish to receive. Many of these organisations accept payments through PayPal.
If you are attempting to find the number of someone who is harassing you, contact the local police, who will also have the resources to track the phone number.