iPhone 3GS not receiving calls when headset plugged in

i'm having problems 3gs. when have headset in, seems ,ot of time calls going direct voicemail. or else if call comes phone, can't answer remote. , if answer call on phone, can barely heard.

works fine when there's no headset plugged in.

i've tried headset , above problems persist. , i've restored hasn't helped either.

ideas?

 

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