Hmm ... try this one: 1. From the Front Screen press the Menu button. 2. Click on the Contacts icon. 3. Keep the cursor on it's Current position and press the Options button. 4. Move the cursor to SIM numbers and press the Select button. 5. Keep the cursor on My numbers and press the Select button.
I don't have a Nokia any more, didn't ever have an X6. You may need to try all the menu trees: but search for a user guide on the Net first, it may be easer to spot it in the user guide.