Whenever I call people using phone, on the other they say I'm soft but when I turn on speaker I would sound louder. Also, it only happen to call, any call with external online app do not have this problem.
Just curious: are you using the noise reduction feature? I'm not sure how this is implemented on your phone, but on many (all?) devices it is done with a second microphone sometimes located along the top edge or the rear of the phone and, with many devices, if the secondary microphone is blocked (by a case or your shoulder, for example), then that will interfere with the noise reduction function and result in low microphone volume as you have described above.