Many people do not understand me well because I'm so quiet ...
Hold the phone closer to your mouth when speaking? Especially if this happens when you use the phone in noisy environments. You can also go to the settings menu and try disabling the (call settings) noise suppression option.
The built in microphone has an automatic level control, so an external mic will do a lot better in unusual situations like this. You can set the mic gain in the service mode menu but it may have undesirable consequences, so you should leave that as a last resort. Moreover you still have to contend with the auto level if you are using the internal mic, so raising the mic gain would not be expected to help if there is high background noise levels.
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