An interface circuit that interconnects a terminal device at an extension station with the adjunct port circuit of a telephone of a business communications system (BCS), such as a digital PBX. The interface circuit enables a call diverted to a BCS telephone to be answered at the extension station. The interface circuit receives a ringing control signal from the adjunct port circuit when its BCS telephone is rung on an incoming call. This causes a ringing current source in the interface to apply ringing current to the extension station simultaneously with ringing of the BCS telephone.