Originally Posted by alalor

If a reset didn't work, something in the connector is making the phone believe you have the headphones in. You first will want to clean the hole for the plug. I would try compressed air first, and then a little brush.
If that doesn't work, you probably have a problem with the contacts of the adapter and the circuit board. For that you open the phone, and check where is the problem, you might have to just clean it, or if a solder is broken, just solder it back.
there is no need to replace the whole phone.