With ISDN the connections are physical: The circuit is brought into your building from the BT exchange by a BT engineer and a small box or a number of small boxes are installed within your building for your business phone system to physically connect to. Each ISDN30 circuit (30 trunks/channels) is one box and each ISDN2 circuit (2 trunks/channels) is a box. For example 8 channels of ISDN2 would mean 4 small boxes being installed.
Scaling up ISDN requires the installation of new circuits and additional ISDN boxes. With ISDN2 you can go up in increments of 2 channels/trunks at a time. With ISDN30 you begin with a 30 channel pipe being installed into the premises and you can 'unlock' anything from 8 channels, as a minimum, up to the 30 channels depending on your requirements. You can also have more than 1 ISDN30 pipe installed if you require more than 30 channels.