Echo problem with Samsung s5 mini dual sim phone

[AC Question]

My husband and I both bought samsung galaxy s5 mini dual sim phones. If we call someone, the other person hears a terrible echo, and we have not been able to get rid of it. There is no "noise reduction" option in any of the settings. We've done a factory reset, turned off Talk Back, and almost every thing we've found online to help. We live outside the US so we can't return them. We've had samsung phones for years and never had this problem. If anyone has helpful advice, it'd be very much appreciated.