How do i see someone's phone number when the person is chatting wid me on facebook ?

[Hoi]

How do i see someone's phone number when the person is chatting wid me on facebook ? Like I recently updated the Mobile settings of my FB profile in which i switched on facebook msg alert n so whenever i receive my FB msgs on my mobile, the msgs comes from an unknown number, So wat i cant understand is it the number which facebook is generating or it belongs to the person i m talking to ? now pls dont ask me why didnt i asked the person to give me his number ? Its just that i want to see the phone number from which he is msging me on facebook. Pls answer, if u know. Thanks
The person's ph number is not given on his profile, is it still possible to see the number he is chatting to me on facebook, what about the number from which i m receiving msgs on my mobile, does it belongs to facebook ? pls help