These are one of the funniest kind of jokes because the Medical profession is otherwise a serious profession.
A Doctor gets taken short up in the middle of the night and finds his toilet is completely blocked.

He says to his wife, "I'm going to have to call a plumber."

The wife replies, "You can't call a plumber out at three in the morning!"

He says, "Of course I can! I have to go out on night-time calls if a patient needs me."

Anyway, he rings a plumber, who complains bitterly about having to come out in the middle of the night.

The Doctor says the same thing, "I have to come out on late-night calls to see patients, why shouldn't you?"

At about 3.30AM the plumber arrives, very bleary-eyed, and the Doctor shows him to the blocked toilet.

The plumber drops two tablets down the pan and says to the Doctor, "If there's no change, call me in the morning!"