Translation:I am on the toilet, can you answer the door?
And I thought the same, for me it looks like he climbed up on the top of the toilet right above the ceiling and he is doing something from there, maybe drilling some hole to install some pipes. But of course as English is not my native I suppose it is normal to say I am on the toilet...
"On the toilet" (as in sitting on the toilet) is a normal expression in English, however, a more polite way to state this would be to simply say, "I'm in the bathroom/washroom..."
"I'm in the washroom, so can you please answer the door". or ..."get the door"
One would never say, "I am in the toilet" as that would literally be interpreted as you somehow physically being 'in' the actual toilet.
The expression "in the toilet", however, can be used to describe...for instance...a bad outcome.
The expression "in the toilet" usually has a negative connotation... such as, "His business is in the toilet", meaning his business is doing poorly or he's about to lose his business.