A number two is code for bowel movement. For health care professionals, it is not uncommon to be so busy that you're either paged while doing a number two or you have to delegate task to someone else entirely, usually an intern or medical student.
Except from Samuel Shem's "The House of God"
Looking sheepish, Bruiser started to slink out, and I asked him if he'd do me a favor.
"Yeah?" he asked, thinking he might redeem himself.
"I've been busy all day and I haven't had a chance to go to the toilet. Could you go for me? A number two. I've done a number one."
More Bathroom Reading