You're going to do this during a call.
During the call,
- You will press the "cid (up arrow)" or "pbk (down arrow)" buttons to display the current volume.
- Then pressing either or those buttons again would bring the volume up or down of the caller/earpiece.
If you don't hear any difference, then the problem is most likely on the caller's end.
**I assume you have a phone that looks like the one displayed in image
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