The RJ9 headset have line order and reverse order. If you connect to incorrect headset, The input of microphone will be small, So the voice you heard will be small or no voice.  

Line order：



Reverse order：

Product model	Order
FIP11W	Reverse order
IP622C	Reverse order
IP622W	Line Order
IP652W	Reverse order

 If the voice is small because of you use the incorrect order, Please change the correct order headset.