My Internal Speaker is stopping during the call

Hey Guys

I bought an used Sony Xperia Z

and its system is Android 4.4.4 Kitkat

the phone is pretty cool

except one thing

when i call someone the internal speaker is working but it suddenly stop and i cannot continue my call unless i open the other speaker

what shall i do ?

is there a fix for it ?

please take in mind : that internal speaker is working but its lagging suddenly .

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