So, call someone, and they answer whilst on the loo.
Who is more embarrassed?
Does the person say where they are, or hope that "noises off" do not give the game away?
What is the etiquette?
Over centuries etiquette has evolved for almost all situations, but now that we have mobile phones, there are new circumstances that need new etiquette.
Personally, I would only answer a call whilst indisposed if I knew the caller would be suitably amused or understanding of the situation.
Even then, if I do not own up to being on the loo, what do I do if the call goes on too long - wait and not flush the toilet. That raises questions of why I did not say earlier.
I really do not know the new etiquette for this. Whilst I have answered calls, today was a case where someone answered a call from me and I was not sure what to say. I said I would call back.