Actually, given that the US's biggest ties with China involve us owing them money I can see a plausible scenario of china calling in its loans, The US defaulting, and China exacting a penalty in the form of demanding (or conquering) American territory in compensation or demanding American compliance as they take Taiwan back by force and make moves to dominate the Pacific Rim.
China cannot "call in its loans". There are specific contracts and schedules attached to all the money loaned between nations. Do you really think governments would enter into agreements to borrow if their creditors could just call in their debts whenever they fancied?
International economics is not like lending your mate a tenner.