How do you prankcall someone on your cell phone?

You don't.
For the following reasons.
1/ it's incredibly immature
2/ even if you withhold your number the network can trace it, both the receiving and the sending.
3/ even if you change the sim it doesn't change your imei.

What is a more productive question is for example "what is the meaning of life?" Although someone who wants to know how to make prank calls is obviously lacking in that department.