With standard (BT) phone lines you are stuck with the allocated line number as your business phone number. A SIP trunk breaks the link between line and number. Your SIP trunk account can utilise multiple numbers of different types, including STD geographic (e.g. 01158526600) and non-geographic (e.g. 0845, 0870).  You can even employ geographic numbers from outside your telephone exchange area, or numbers from a different country, which is ideal for marketing campaigns and International satellite offices.