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That could be a network issue. does it happen with every call or just an specific number?
That's quite odd, what if you restart the phone? Do you have a NANO SIM card or do you trimmed a regular SIM card?
Do you have a case on the phone at all?
Please confirm these as well:
An echo may be heard during a voice call if:
1. Large amount of data communication during the call.
2. GPS is accessing data during the call.
In this case I would have to advise you to speak with your local Xperia Care team regarding having an assessment performed.