11-06-2013 20:46 - edited 11-06-2013 20:48
Is the behavior the same when you have a headset connected to the phone? If all settings are properly setup, the issue may lie on the hardware already, but you can still try performing the troubleshooting steps provided in this link to further identify where the issue lies: http://support.microsoft.com/kb/2606900
If above link wouldn't help, it's best to bring the phone to a Nokia Care Point for assessment: http://www.nokia.com/global/support/locations
Had the same problem with my Lumia 900, so sent it back for repair then 1 month later it does it again!
Then just today I notice a piece of paper stuck in the earphone jack so I took a pin to it and contiued picking out pocket fluff it kept coming there was loads of it. Can now make calls without loud speaker!! I can only assume that my phone thought i had earphones plugged in. worth a try