If you came to know your mobile is missing, do not get tensed; there are high chances that you will get your cell back. first you need to do is normally search near by, if you find your mobile thats great if not really, not to take pressure and sit quitely and believe when have you made any call last and search at that position.
Trace Mobile using IMEI
IMEI is usually 17 digit number which is assigned to a cell by its vendor which you can use for tracking mobile.
Now you need to contact your cellphone providing organization and they can help you discovering your mobile phone.