If the receiver playback test (in CSD) fails then the so-called receiver or earpiece (the sound-producing component at the top part of the phone, that is supposed to be next to one’s ear during a call) might be defective. In particular, if you can never hear what the remote party says during a phone call (but you can hear that via the HF loudspeaker at the lower part of the phone).
Sorry, that is just bad language. It means the microphones at the top and bottom parts of the phone.
Well, that is from year 2016. Sailfish has changed a lot since then, and there are new devices in use. But if there is an audio routing error somewhere, connecting/disconencting a headset might help to correct the routing.