Dynamic address assignment means that the device must ask the network for an address before it can perform any signalling.

The Q.921 standard (also called LAPD) that defines the link level protocol used on the D-channel incorporates a mechanism by which the device can request an address.

In this case, the device which currently has no address is able to request one from the network. The network responds with an address for the device which it will use for all future communications.