Microphone issues

I bought my phone on christmas ( iphone 5s). First of all it was amazing then the quality of the microphone decreased linearly and my all my contacts couldn't hear me very well till now i have the same problem sometimes i have shout so the other person could hear me.

i Bought my iphone from UAE but now i am in lebanon also i contact my career services they check my line i don't have any problem

so please help me to know what to do thank you