Took me to the attorneys after settling an account after 3 months

I lost a Job June 2017 and i had a balance of 150 outstanding on the account for On Air. I have been paying them although not working. I asked for a statement as they said i ow them 150. It took 20 days for them to come back to me which was by August which i ended up settling after getting a statement. Lately I got a call on the 11th of this month (November 2019) from the attorneys saying that i owe them R150 which i don't understand