Money At Call
  
We give you a large duffel bag full of cash to hide in your attic. We also give you one of those burner phones you can buy at Walmart for $79.99. If we hit you up on the burner, you have to bring us the money immediately.
That situation represents money at call...literally. It also describes the normal everyday definition of the term.
The phrase usually denotes a loan that can get called in (hence the name) at any time. When asked, you have to hand over the full amount owed at the moment.
Money at call typically applies to short-term, bank-to-bank loans. One bank will loan another one money, sometimes just for a day or two. These quickie loans usually have a money at call provision.