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What is the model of your headphone? It sounds like you are using a headphone with OMTP standard. Check the following link for further information:
http://talk.sonymobile.com/t5/FAQ/Problem-using-headset-accessory-not-supported/m-p/347872
Note that even if you use a headset/headphone with the CTIA-standard, it can in some cases need more power than the phone can deliver. Thereby, we can only guarantee that a headset from Sony is compatible.
