The present document defines the stage 3 specification of the Supplementary Service Ambience Listening (SS-AL) for the Terrestrial Trunked Radio (TETRA). Stage 3 defines the signalling system protocols and switching functions needed to implement the service described in stage 1 and stage 2. Charging principles and Man-Machine Interface (MMI) are outside the scope of the present document. The SS-AL enables the served user to place a TETRA Mobile Station (MS) into a special type of voice call whereby the called MS transmits without any action from, or indication to, the affected user. The SS-AL call can include a second listening party. The affected user can