Abstract: In a telecommunication system, a circuit is provided for determining the system mode and for restoring converted signalling bits. The circuit is designed to be used in a telecommunication system that lacks frame alignment. The circuit may serve as an trunk line interface located in a Direct Digital Access Office Channel Unit. Alternatively, the circuit may be coupled to a single trunk line which links Customer Premises Equipment (CPE) to the telecommunication system. The detection portion of the circuit determines the system mode of the trunk line connected to a CPE by detecting signalling bits when frame alignment is not maintained. The restoration portion of the circuit restores signalling bits which were converted due to a lack of frame alignment to their pre-converted state.