In one aspect of the invention, emergency modes of operation for an adapter unit comprising a channel transmitter/receiver unit, a data packet transmitter/receiver unit, and a data insertion/data retrieval unit which inserts signaling data transmitted by the channel receiver unit into data packets and forwards the same to the data packet transmitter unit while retrieving signaling data from the data packets received by the data packet receiver unit and forwarding the same to the channel transmitter unit is provided. The adapter unit especially allows terminals of a branch to be connected in a fail-safe and simple manner to a central telecommunication system.