How can i deal with NCO financial if i don't have a debt?

I have been getting phone calls from 8002400381. First time i didn't answer, then i looked up their number online and found that they are a debt collector company. i know that i don't owe any debt. So i didn't answered the last 2 calls from them. i think they will keep calling me. How can i deal with them in this case? Can this be a sign of identity theft?