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.
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