After getting back the phone, I tested without any casing and my voice came out clearer than before (also without casing) on the other end of the line. So, to a certain exent, the repair did improve the call quality.
When I put in the Rock PU leather flip casing, the problem came back. I changed the casing to the Nillkin matte hardcover and the problem disappeared. Then I realized the difference in the design of the camera/flash opening for both the casings. Rock flip casing covers the whole protruding camera assembly except 2 small holes for the flash and lens. On the other hand, the Nillkin casing exposes the whole camera assembly in a long rectangular hole opening; where you can even see the words 'Carl Zeiss'. For Rock flip casing, these words are covered.
Under bright lights, I can see a small dot in the gap between the body and the camera assembly (below 'Carl Zeiss'). To test it, I go to video mode and blow softly at the camera near the dot. When I play back the video, the loud whooshing sound confirms that the secondary mic is near the camera assy; and this mic is used for noise-cancelling during normal voice calls. I suspect the exact location of the mic is around the dot below 'Carl Zeiss'. With the Rock flip casing, this mic would be covered up and caused the voice call problems.
I am not sure if the Rock casing design is similar for L620 and L920. Not to mention that the camera assy design and secondary mic location are also different for the other 2 Lumias.