In addition to fixed and dynamic addresses, there is also a universal address. This broadcast address is used when a message has to be heard by all devices sharing a given D channel.

Messages sent by the network to the broadcast address are heard even by devices currently without an assigned TEI.  

Broadcasts are mainly used by the network to signal an incoming call - that way, any device connected to the line can take appropriate action - an ISDN telephone can ring, a TA could make attached analogue phones ring, or a device could decide to answer the call.

However, before a device without a TEI can answer a broadcast (to answer an incoming call, for instance), it must request and receive an address in the normal way.