Contacting Dr. Hayne's office is one of the requirements you may be presented with while watching Black Mirror: Bandersnatch. This page of our guide informs about the correct number you must type in so that Stefan can make the call.
Using the phone to call the office of Dr. Haynes is one of the well hidden choices. This usually occurs after Stefan has opened the safe in Dad's room using the PAC password and uncovered documents on himself.
This step is very unusual, because instead of picking one of two available options you must manually type in a five digit number. You have 10 seconds to do it. The correct number is 20541 - it's also hinted by the characters seen while the clock is ticking waiting for you to input the right combination.
You have only chance to pick the phone number. If you get it wrong or don't finish before the 10 second mark then you'll fail to make the call. Stefan should then proceed with getting rid of his father's body.