Why do people say that China can call in the US Debt whenever it wants?

Because they are misinformed.

Because they're ignorant
China sold that debt long ago

Because "people" are stupid. Let me turn your attention to the answerer "sherly". She/he's a PRIME EXAMPLE of STUPIDITY

China can stop purchasing US Treasuries whenever it wants. That has the same effect.

China can sell Tbills and the U.S. must pay their value. I guess that amounts to the same as calling them.

China sold all their us debt. It’s now owned by Americans and the Japanese.

First, China has so much of it that every day is the expiration date of some of it.

Second, it's transferable. They can sell it. Putting even a small part of it up for sale would have disastrous consequences for the global currency markets. And the U.S. economy..

Because they can we are in debt to them