Help Echo sound

[RSSD]

Hi there,

my wife and i both have the same samsung galaxy ace 3 since about a month now.
Last week my wifes phone suddenly has a weird echo sound, only heard by the people who call to her phone.

It is really annoying and i can't find out what is wrong.
I tried a lot of things allready, reboot,reset, nothing seems to work.
I allready tried the app that i found through this forum, but that allso didn't work.

Any ideas ?

Thx in advance.