seat belts gets locked and won't come out

Both front seat belts gets locked randomly without warning , contacted Mercedes , they told me to replace seat belts but won't guarantee if that will fix the problem , the car will act normal for days and then suddenly you come back and try to put seat belt and both will get locked. Mercedes engineers need to bring it back to their factory and find out why the safest car on the road so as they claim is acting so Unsafe.