If the person on the other end says that he/she can't hear you, check whether the microphone works and then try the following.

iOS

• Go to "Settings" on the device > "SkyPhone" and turn on "Microphone."

Android

• in SkyPhone > Turn on "Mic Boost."

Both OSs

• Turn on the "Volume Up" button while calling.
  Note: The sound quality becomes inferior.