TELEPHONE ETIQUETTE: Is the use of words lavatory/latrine impolite/improper?

David is a meticulous person. He is formal, planned and serious in his manners.

Rabby is his roommate. He is flexible, informal and jovial.

When David was in the toilet, his cell phone rang. With no prospect of David returning, Rabby took the phone and replied: "David is in the lavatory. I shall pass on the message to him. If you do not mind, kindly ring after ten minutes."

The caller thanked Rabby and rang up David after ten minutes. David already back from the toilet, and attended to the caller. Their work was complete.

Now, the postmortem started.

David: "You have used the word 'lavatory' which is an unparliamentary word. In future, whenever you reply my calls say that I have gone outside or that I am not available."

Rabby: "The word 'toilet' seems to be more popular. Yet there does not appear to be any taboo or prohibition formal or informal on using the words lavatory and latrine. I wish that I always tell the truth, using the words which reflect the truth as much as possible. In future, I shall reply that you are in the toilet. If I say that you are not available or have gone outside, the caller may answer further questions. This increases his phone bill. Also leads to waste of his time. Anyway, what is wrong in giving exact details?"

David: "There is nothing wrong. It looks odd."

(Comments on the above conversation are welcome.)