Talk or write them with the problem and ask them to explain. If you've been ignoring them for a while, they will file suit. If not, they might find communication cheaper than litigation. But, if not, you'll find out what they're talking about if they file a case. You can pay them then. (If the debt is yours.)
When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.
--W. T. Pooh (aka A. A. Milne)