Why Consider ISDN?
In the mid 80's ISDN was originally conceived as a high speed digital alternative to analogue modems. Prior to the arrival of broadband, ISDN was the only way of achieving higher transmission speeds without a dedicated leased line. Now there are over a million broadband subscribers in the UK, the outlook for ISDN was bleak but ISDN has fought back with some very clever ideas such as pooling of lines and direct dial to the desktop.
The main drawback of analogue is that you have to install and pay for each line which runs to a dedicated device. So if you need four voice lines, two faxes, a RAS connection to your server and an alarm that's eight lines that have to be paid for. With ISDN you estimate the maximum concurrent requirement and then pay for just that number of lines as all the devices are served from a pool of dynamically allocated lines.
ISDN has other tricks up its sleeve such as being able to run different businesses from a single phone and know which business any incoming call is for making it ideal for serviced offices. Voice quality is clearer and Least Cost Routing becomes easier to manage as the PABX (Private Automated Branch eXchange) flags the call before it leaves the premises.