Saw a bunch of posts on this and some of the answers are pretty stupid. In fact, some are malicious. There is no software reset, reboot, network reset steps to do this. The headphone jack is DIRTY and/or got wet. That simple. Phone thinks there is a jack in there so its stuck in headphone mode. One suggestion was to stick a paperclip in the jack. That will potentially damage your jack and phone!!! How stupid can one be?