China owns something like a third of the total foreign held US debt
The amount of US debt China owns is less than 10% of the total amount outstanding. Currently around $1.3 Trillion which is a big number but only a single digit percentage of the total debt. Most of the US debt is actually held by Americans. Of the $12.9 trillion chunk of debt owned by Americans, $5.3 trillion is held by government trust funds such as Social Security, $5.1 trillion is held by individuals, pension funds and state and local governments and the remaining $2.5 trillion is held by the Federal Reserve. Basically most of the debt is IOUs to the American people.
Interestingly Japan owns almost as much US debt as China does at $1.1 Trillion. But Japan isn't so scary so people gloss over that fact.
Although China needs the US as badly as the US needs China, if we try to bluster our way into something stupid, just calling the debt will make for a rather unpleasant time as the world economy topples.
China has no ability whatsoever to "call" the US debt. Treasury bonds don't work like that. China bought those treasuries to keep their currency exchange rate under control. Furthermore even if China wanted to get rid of their US denominated debt, they have absolutely no one else they can sell it to. There simply are no buyers for that much US debt at anywhere close to face value. If they hold a fire sale they absolutely screw their own economy in the process.