When I have heard the echo, it is from the cell system itself, not something going on the in the phone. I don't know of any fix for it as it related to the cell system and the provider there.
For me, echo varies from location to location and is not permanent, just occasional. If you are hearing it more often, I would call Apple about getting a replacement. And yes, I mean the cellular provider and the technology of the towers through which the signal passes.
Are you talking about hearing yourself echo back as you talk? If so, 99.9% of the time (in my experience) that means the person you're talking to is using a Bluetooth device to talk to you and as soon as they switch to the handset instead, you no longer hear yourself echo.
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