I know this is a good while now but here is my 2 cents worth as i have encountered this a fair few times now. i have found that the echo goes away when battery cover is removed but reappears when pushed back on. i wanted to get to the bottom of this as it did not seem like a hardware failure.
firstly (the obvious)remove any protective case and check if same issue.
then for only testing temporarily!! ,try sticking a pin or covering up best you can the tiny hole for top mic which is located beside earphone jack.
if you find the echo goes away then try next step below (you dont want to permanently completely block this hole as you will have poor speakerphone and video audio quality).
The proper fix would be to put a thin layer of foam in between the top mic and black rectangle mount shown in image .
it can be done carefully with only having to remove battery cover. a thin screwdriver in between these areas will prise and give some slight access. Best to work from left hand side of top mic incase you rip cable on right hand side
i would advise trying this before even looking at a full headphone jack or dock replacement