I've had that, like I will call someone (or they call me) and I put my phone on speaker and all of a sudden they can't hear me. I don't know what causes it but it's a pain to deal with. It also happens if I have a headphone device plugged into the phone, they hear me fine but once I remove it they can't hear me. Then I have to plug the headphones back in and unplug them for them to hear me. I have no idea why these issues happen.