Incidentally, it's not just ISDN that is being retired it's all of BT's traditional switched network - its Public switched Telephone Network, the PSTN. This is the everyday service that everyone uses in their homes and small businesses. So pretty soon everyone is going to have to switch-over too.
If you need a new PBX, are replacing an old one, or coming to the end of your telephone contract you should not continue to use ISDN.
The replacement for an ISDN circuit is a SIP trunk (Session Initiation Protocol). SIP is the specific technology that connects your telephone or switchboard to a Service Provider - such as Voipfone - so that your calls can be routed to their destination. A technology called VoIP, Voice over Internet Protocol is used to carry you calls.