Nokia Lumia 510 - Why am I unable to hear properly the other end on my Nokia Lumia phone?

The volume of audio during a call has a separate setting between 0-10 (“0” being “off” and “10” being max) which is independent from the device audio setting (which can be set between 0 and 30).

You will be able to control the volume by side keys while in the call screen. To make the on-screen volume indicator appear, take a call and then press the volume up once you hear the other end, the optimum volume level is 5/10.