Where did you initialise serial data speed on Bluetooth device?
Cyclegadget is right - it's either when pairing or whenever connection is established
Even if baud rate was wrong you'd still receive something - if you don't then it has to be something else.
Verify if the module and the mobile works using a USB bluetooth dongle and a program like putty. Also check the datasheet of the module and the settings - is the module set as slave or master ? For apps I created it has to be set as master