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.
Cost savings, however, are not the main reason for choosing
ISDN, so what is? Follow these links for the answer to that
question, Advantages
of ISDN. and ISDN
in English.