Types of Lines
Any business purchasing a new telephone system today should not really consider analogue lines at all. Despite what most people believe ISDN lines are virtually the same price as analogue, the only point to consider is that they have to be purchased in pairs (up to 8 and then they can be increased individually). So as long as the number of lines required is an even number (if below 8), the cost (apart from installation), i.e the line rental and call costs is all but the same, the bottom line equates to a few pounds per YEAR.
In fact the overall cost of running your phone system with ISDN lines is almost always less per year than analogue, as other devices like fax machines and computers can also be connected through the PBX, so the total amount of lines rented from BT will probably be less than it is currently. This is because devices such as fax machines are generally much less busy than voice lines and do not need a dedicated line each.
It is also possible to retain your current telephone number (as well as getting an unlimited amount of new numbers...see DDI) and receive much much more functionality than is possible via analogue.