Statute of Limitations on Debt Texas

We just received a call from a collection agency saying that my wife owed them $100+ for blood work done for a doctor in 2003. At the time she was covered by insurance. The SOL for Texas on debts is 4 years. The collection agency first said that an attempt had been made to contact her over the years (which was not done) and later said that the bill had laid in a file and had recently been discovered. My wife no longer has that insurance company or that doctor. She told the collection agency that she is not going to pay anything without seeing some proper documentation of the bill. We are very skeptical since in 2004 after her mother died, we started getting calls from many very questionable collection agencies about bills that her mother was supposed to have charged after she died