I have encountered this issue two separate times with my Lumia 925. I sent the phone in for warranty repair earlier this year and they sent it back in working condition. However, they didn't specify the repair, so I'm not aware if they replaced a hardware component or if they just reset the phone. The same device (IMEI number the same) came back, but it was reset to factory settings. A few months later (i.e., last week) the issue came back. Basically, the phone thinks the headphones are still plugged in (when they aren't) and I can't take any phone calls, unless it use the speakerphone. What is odd is that I can get it to behave correctly if I play with the 3.5 jack a few times (insert the headphones and pull them back out again). However, the phone will then default back to headphone mode at a later time even though I don't insert headphones again. Moreover, when I do use the headphones, sound comes out of only one side. I tried upgrading to 8.1 as well as a factory reset. Unfortunately, neither solved the problem. I just sent a note to Nokia, again, regarding the issue. I have owned 4 Nokia WP8 phones and all have had hardware issues. I'm not impressed.