This is an easy joke to play and it can be very funny

Step 1: What You Will Need

Someone's phone do not steal it: Borrow it or "Find" it.
Common sense

Step 2: A Number

You can put in this person's parents' number or a friend from their contacts list or a number you want to put in like your mom or dad or someone you know who is grouchy.

Step 3: Change Other Numbers

Choose any random people and Change their number to the number you have, Or you could choose people strategically like their girlfriend/boyfriend or anyone else they would phone a lot.

Step 4: Optional

If you want to see your plan working and be able to laugh at the person you might want to use your number instead of another persons number.
You can also put someone you want to annoys number in there and they should get a call from the same person now and then