If you never got anything in writing from them, don't worry about it. Next time they call tell them they call, tell them they already violated the law by not sending something in writing and if they want to keep racking up violations, tell them they better have their records in order.
Get their mailing address if you already don't have it and send them a debt validation letter, certified mail return receipt requested. Ask for basic information such as full account history, original signed contract, the right for them to collect and so on. there is a sample letter to use in the do it yourself section at the top of this site.