Try disabling noise reduction in call settings... echoing has been a problem since the s2 sometimes disabling noise reduction helps or turn down volume on your phone.
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AT&T SGS IV
Nottach Mint Jelly
Ktoon TW JB 4.2.2
Phones that came, flashed, and gone:
HTC Touch Pro, HTC Touch Pro 2, HTC HD7, LG G2X, SGSII, Motorola Atrix, Gnex, SGSIII, HTC One
If I get a million thanks my wife says I can have any phone I want! Hit thanks if you have a heart