How can you tell the shortest way to the next emergency telephone on federal motorways?

You are correct. The shortest way to the next emergency telephone on German federal motorways is indicated by black arrows on the white road delineators (the posts at the side of the road). These arrows point in the direction you should walk to reach the nearest emergency telephone, which can be especially important if you need help and your mobile phone doesn’t work. The guardrails and ground markings do not provide this information. Always look for these arrows on the delineators if you need to find the emergency telephone quickly.