Yep....dispute the debt, get validation.
Specifically request a copy of the pay history, balance sheet & payoff breakdown along with a copy of the signed contract.

FYI: MO has some of the best post judgment collection remedies available to creditors. Wage Garnishment, Bank Levy & liens on realestate to name a few.

Verify your last paid date and type of debt it is, so you can find out what the SOL is (I can't recall what it is at this time). Depending on the type of debt, the SOL may vary.

I only agree with Woofer on waiting it out to the extent they validate the debt. Waiting it out in the hopes that you'll get a better deal later is risky and could potentially cost you more money.

Plan B: If they respond and validate your debt like they should, consider finding another lending source and try to settle and if they refuse to settle (again, after validation) pay em' off and be done with them.

It's too bad that your dealing with a creditor that gives you long hold times and by the sounds of it, making it difficult to resolve the matter voluntarily.
Very frustrating.