Yes. Write them a letter stating that you do not know what the amount is regarding and you cannot take any further action without knowing full details of the debt. You can ask for a copy of the agreement that they believe led to the debt.
Keep a copy of all correspondence and, if you don't get a reply, send the letter by recorded delivery next time. You must reply to the correspondence. Once you know more details, you can consider other options. If you get no further correspondence after sending a letter, you can forget about the matter.