[Tribanj]

On my Nokia Lumia 735 when is call, I hear the caller, and he does not hear me. What's the problem?

[Tribanj]

When I use Speaker, my interlocutor hears me.

Zvučnik = Speaker

[Harrie-S]

Than you have a problem with the "call" microphone of your phone. If you use speaker mode your phone uses another microphone. Your phone still should have warranty so send it for repair/replacement.

[Tribanj]

Yes, but sometimes works and without the use of the speaker.

[Harrie-S]

Then there are at least 3 possible explanations.
1 The "call" microphone has a "loose/bad contact. (send it for repair/replacement)
2 The phone "thinks" there is a earphone plugged in the 3.5 jack. (dirt can cause it) Send it for repair / replacement / Clean the jack good with for example a dry toothbrush)
3 A software problem. ( Possible solutions are : soft reset, hard reset, Lumia software recovery tool)