Virtual numbers can be non-geographic, such as 0800, 0845, 0870, these are not linked to a physical phone line; they are forwarded to your phone lines.
There are a number of reasons for businesses to use non-geographic numbers for example it makes office relocations or expansion very easy. The worry of communicating your new number to all of your contacts is removed. Needless to say, the possibility for missing those potential clients who have your old number would not exist.
On the other hand your business might benefit from a virtual geographic number such as a London, Manchester or Glasgow. These allow you to have a local landline number for an area even if your business does not reside there.
Virtual numbers have the added advantage of being able to track inbound enquiries from marketing campaigns, thus providing real time statistics and increasing your return on investment.