I usually use the headphones for calls, but had unplugged yesterday and did not notice a problem at first. I got a call, but could not hear anything on the handset (the schematic calls the in-phone telephone speaker the "receiver"). I assumed it was a connection problem on the callers end. A few hours later, got a call, picked up, and again, no sound...after repeating, "Hello?...hello?" a few times, I hung up. Again a few hours later, my wife called. When I saw her call, on a hunch, I hit speaker and we spoke normally. Now, my suspicions raised, I started to test various apps and did a test call with skype and realized that the receiver would not work.
Fast forward a half hour or so of reading various forum posts and I read that the problem is sometimes called being stuck in "headphone mode." All kinds of solutions were offered, from "jacksex" (essentially just pushing the headphone jack in and out, in and out...) (tried, didn't work), restoring the phone (tried, didn't work), to sticking the phone in the freezer (didn't try, though there seems to be a large segment of people who believe the problem is related to phone overheating), to....drum roll please....blowing as hard as you can into the empty jack. I am happy to report, after some real frustration and nightmare images in my mind of having to pay for a new phone, that this solution fixed my problem.
Thought I would share my low tech solution and triumph over this problem! Not sure how prevalent this problem is but after a few hours of frustration, I figured I needed to share somewhere!