Samsung Note 3 Microphone Issue: Inconsistent Call Quality, Speaker Mode & Coverage Reception

At the beginning I would like to welcome everyone and apologize if I have mistaken the topic in the forum but I rarely come here.
Dear forum members.
I bought a used samsung note 3, unfortunately the previous owner did not say about hidden flaws. After the purchase I had to give the phone for repair because it caught the coverage poorly. The commission said that the phone was flooded but I do not believe it because the coverage as it was once is full and once it falls to one mark, for repair I gave PLN 130 and the meatloaf unchanged, there is one more problem, namely, when I call someone I cannot hear me but I will hang up and call again it is OK, there is also a problem that if I talk to someone and turn on the loudspeaker mode then the person on the other side at all he can't hear me.
A guest from the commission said that the motherboard is to be replaced but, but all functions work perfectly, buttons, wifi, bluetooth modem from the internet, it all works great.
In this case I have some questions. Dude pulled me on cash?
how many microphones are one or more?
and the last question is whether you can change the board with a USB connector where the antenna (would improve the range) and a microphone that would work with the loud-speaking function. Note 3 has Android 5.0 from the plus network. (It was bought on this network.)
Thank you very much and I am asking for help.
Greetings.

Note 3 has three microphones: one on the USB board for calls and two on the motherboard: one for hands-free calling and the other for reducing ambient noise. You can check the correct operation of the microphones by entering the code * # 0283 #. If the phone was flooded, then the lottery - some faults may come out after a while, if you have the opportunity, return it. The USB board does not affect the range, damage should be sought on the motherboard and I guess it has to do with water damage.
Greetings.

Hmmmm.
Unfortunately, I can't return.
And the phone after a fall with a cracked display may have a damaged motherboard?
can he buy one and change the disc itself?

The discs tend to withstand falls where the lcd breaks. So a good chance you'll buy sure.

Well, rather.
What do you advise in this case?
I will add that except for the disadvantages and the fact that sometimes the range will drop to one line (not all, you can call) this note is ok, everything works smoothly and nicely as if nothing happened.

Can set the network type only to GSM. Maybe some antenna connector is broken. Most likely the microphone to be replaced.

Thank you very much for your time and answers, I decided to buy a bluetooth earphone.
So the topic can end.