Samsung flip phones typically offer volume buttons on the side of the phone. When disconnected from a call, these buttons control the ringer volume. However, when connected to a call, these buttons control the listening volume, so you can better hear your caller. The volume level chosen persists between calls, so you do not need to reconfigure the volume for each call.
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Open your Samsung flip phone.
Make a call. If you don't want to call an actual person, call into your voicemail.
Press the upper volume button repeatedly until the desired volume is achieved. The volume button is usually located on the left side of your phone when looking at the screen.