"Has anyone ever considered what will happen if China calls its loan?"

what loans are you talking about?  

You mean the US Treasury Bonds they've bought?  Those have maturity dates.  One doesn't "call them in".  

Plus they are denominated in dollars.  Guess who prints dollars?

And what would happen if China sold that Trillion dollars of US debt they hold?  Well, the Yuan would rise and the dollar would drop.  Making Chinese exports less attractive, and US exports more attractive.