Here are some useful links:

http://www.magsys.co.uk/telecom/codelook.asp
Code Look has two very helpful functions, firstly, looking up partial telephone numbers to find the locality and tariff band, Secondly, looking up localities to find the code.
The Code Look database is updated approximately every one or two weeks making it a very useful website.

http://www.sipknowledge.com/SIP_RFC.htm
Sipknowledge.com has all the information on SIP. If there is anything you want to know about SIP, just go to this website

http://www.localcallingguide.com/lca_prefix.php
Local Calling Guide – Area Code/ Prefix Search