I spent the last 2 days trying to find a simple solution to this problem. Even though my phone is still under manufacturers warranty and Service Providers warranty I decided to just do this as working carefully there will be no indication that the phone has been opened or the "fix" has been applied
I for one couldn't take my phone to a service centre as they would have booked in my phone and gave me some Nokia 3100 knock-off budget phone for one and the repair would have taken 21 days and having business requirements that my phone needs to meet I decided to just do it myself.
So I decided to open it up myself and fix it and here is the guide on how to permanently fix the echo that people get when phoning you.
When you receive or make a call people on the other end of the line hear a echo of themselves and this might be intermittent but seem to be worse when having any type of cover on the phone.
The reason this occurs is because the speaker depicted in the 3rd photo labelled as "B" has a rubber seal at the bottom of it that has gone loose from the casing by either a drop or some vibration that is making the speaker's sound go into the phone and picked up by the NC (Noise cancellation) microphone shown in the same photo as "A" rather than be isolated and out the speaker hole on the front of the phone.
This fix is fairly simple and permanent. You only need a 5mm x 12mm piece of cardboard or think paper that is around 0.5mm thick that can be used as a buffer.
Picture 1 Shows the phone screws that should be removed 1st depicted by round red circles. Keep the screws in the same order as you remove them as only 4 of the 7 screws have locktite on them and be sure when you replace them you put them back in the same position.
Picture 1 As depicted from 1 to 6 is the easiest way to follow to unclip your back and front cover from one another to not break any of the clips. The back cover is very rugged so don't be afraid to unclip it as it is unlikely to break.
See Below Picture no1
Picture 2 is how it should look like when the back cover has been removed. We will only be focused on the area in the red block called ZZ
See Below Picture no2
Picture 3 shows the 2 components that's interfering with one another. "A" is the NC microphone sensor. Be sure to check that the sensor is properly inserted into the rubber sleeve covering it. "B" is the speaker and is glued with some ridiculously little amount of glue into the housing of "B" and can be lifted with a tweezers or some small object to be able to lift it out of the housing in "B"
See Below Picture no3
Picture 4 Loosen the speaker by just moving it in this aspect north and south a bit until it can be lifted up. Please note the arrow as it flips the way the arrow is pointing to expose the casing it is housed in. Clean out the housing of any dirt and wipe the speaker off as well of any dirt or substance that might elevate the speaker a little bit in its housing.
Picture 4 Flip the speaker back into its housing and firmly press it down so that it seats itself well in the housing. The rubber that the speaker is surrounded with should press firmly in the housing to seal it of any noise coming into the phone.
See Below Picture no4
This is where you use the piece of cardboard or thick paper of 5mm x 12mm by 0.5mm thick and place it over the speaker. I recommend cutting up a business card as they are the correct thickness. When looking at Picture 4 the cardboard should be covering the whole red block as depicted by "C" and should look like Picture 5 when placed
See Below Picture no5
Close up the phone and screw everything back together. The piece of cardboard or thick paper will ensure that the casing press the speaker tightly in place and keep it there without letting it leak into the phone to cause the echo effect for people calling you.
Enjoy calls without any echo effect.
This is all very simple to do and my echo has gone away completely. The replacement part they are referencing to in other topics is not really the problem and has nothing to do with it. In the process of replacing that part the speaker gets loosened and removed out of the housing and upon putting the phone back together they securely glue the speaker back in place and thats why they get the same results.
If this guide helped please post and say so. The problem has no relation to software or anything other than the speaker leaking sound into the phone being picked up by the NC.
Hope this helps. My phone is working 100% without any issues.