Return of the Pink Panther - He Said He Was a Repair Man
When the inspector came to the house, he said he was a phone repair man, and he asked to see the nearest phone. The 1st one would not do, so he went into the study to use that phone. But he overheard a conversation between the butler and the woman, and after the conversation, they called him "inspector." How did they know he was an inspector?
If you watched closely, the man and woman snicker to each other when they first see the inspector show up at the door as the telephone repairman. They know who he is. That's why they "stage" the conversation on the telephone.