I received a paid up letter but I'm still being called.

<p> At the beginning of August 2015, I called Capfin wanting to settle my account I was told I was handed over to Vertex solutions. I called Vertex and I was given a settlement amount which I paid within 7 days of it being given to me. I even received a paid up letter in this regard. </p> <p>To my surprise at the beginning of this year I got a call from ***, calling about this very same Capfin account. I called Capfin and even sent the paid up letter and I was promised this would be sorted out. Up until now nothing has been done as last week I received a call from the very same company telling me the amount is needed asap. What am i to do? How many accounts did I have with these people? Even on my credit status this information has not been updated. </p> <p> </p>