Sm 900p others can't hear my voice. Signal falling to no mobile network available

Is there anyone who can answer my query. I bought a samsung galaxy note 3 sm n900p from eBay India. Everything is working fine but the main thing calls are not working properly.. The other person can't hear my voice 4 out iof 5 times. Signal getting cut all of a sudden to no service available and in the next second and then again gets the signal. What should I do. I tried factory reset, cache partition wipe and what not but nothing worked. Can anyone help. I like the device like anything. Please help. Device hardware is working wonderful. No problem in it

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