Ask your phone company. They put the number on the SIM-card. It's not something you can do yourself.
Quite understandable, by the way. Wouldn't you love to have the same phone number as, say, Brad Pitt or Angelina Jolie or Barack or Michelle Obama? It would be quite confusing if 1 million people did that, both for the person who phoned them and for the cellular networks.
Is there any way to have the same number working on 2 separate sim's simultaneously so that i can use the same number on two phones. Is this legal? If there is any way how to do it?