Harrasment from NCA

I had over 20 calls to my job today in a 5 minute period. I kept just hanging up. I sit in close quarters with other people in my office and there is no way to have a private conversation, so I finally answered and lied that I no longer worked there. My cell phone immediately started ringing, so I stepped outside and called this guy back. I was on the phone for over 15 minutes. I told him to stop calling that I did not owe for these PDL's and had legally paid everything I owed. I asked him to send me proof of what I owe and he said no that I was just stalling and they had already sent me info several times (to my knowledge I have never received ANYTHING from them in writing). He kept threatening to sue my, get a garnishment, etc. Refuse to give me any info other than his name and company, and the PDL's he is collecting for. He also called me a deadbeat and fianlly hung up. Help! I am afraid I will lose my job if he continues to harrass me this way. What can I do? I just filed a compaint with the Oklahoma Department od Consumer Credit and am sending a debt validation letter today. What else?? Will they still call anyway after receiving the letter?