@cicerone imposter,
China cannot cash in US Treasuries all at once, ie. "call in the debt". The Treasuries are purchased at a certain date, and the due date is some specific date in the future. Before that due date, the buyer is not due a nickel. At the due date the buyer cashes in the Treasury bill and receives the amount the T-bill was worth, which is always more than the amount the buyer paid. China can only redeem the T-bills on their due date-they cannot walk up to the US and say, "Gimme my money for all these T-bills I have". They will receive their money for the T-bills that have come due, they won't receive any money for the T-bills which are not due yet until the due date arrives for those bills.
Paying off the T-bills on their date is known as "debt service", and it takes less than 10% of Federal revenues per year. That leaves over 90% of Federal revenues to be used for defense, health care, etc. In short, debtwise the United States is in pretty good shape. There is no crisis at all, contrary to the right wing media.