This statement is factually wrong. Even if it were possible, the only way they could attempt to enforce this would be to confiscate American assets in China. In which event we would probably begin seizing Chinese assets around the world to compensate the US corporations affected.
I misused the words “calling in” in this context, when what I intended to say was “selling”. China currently owns over $1 trillion in US debt in the form of bonds of various maturities, which it can theoretically sell any time it wants to whoever will buy them. However, mature debt securities must be accepted and redeemed by the issuer (the US) or else offered to be rolled over upon new terms acceptable to the buyer.