A bunch of *****

I was on One Plan over a year ago and now I received a message from a lawyer saying that they have handed me over for not sending them a invoice in February 2018. I get told by the lawyers from Nudebt that they have tried contacting me numerous times but I was only handed over 7 days ago. I called One plan and they said that the last time they TRIED to contact me was in February 2018. I would like to now how is it possible for anyone to say that this was numerous times. I then get told that they did receive the receipt from the doctor but was not good enough because the doctor did not stamp the receipt. I would like to now if anyone can help me with taking this matter further.