Why can the other side of the call not hear my voice when using a headset? - Nokia FAQ
In case of audio issues, check the following:
Check that you are using a compatible headset, e.g. the headset included in the product package.
Check that the headset is properly plugged in your phone. Be sure to press the headset to the bottom of the connector.
Check that you are speaking to the microphone. If your headset does not include a microphone, you need to use the phone microphone. Keep the phone microphone close to you when speaking if there is no microphone in your headset.
Check that you have not muted your outgoing audio. E.g. on the touch screen devices, check that the Mute / Unmute toggle switch on the screen is in proper state (showing Mute command when outgoing audio is enabled). If you have by mistake muted the outgoing audio, tap the Unmute toggle switch to enable your voice to be transmitted.