Who called from 2012795778

recieve between 10-20 calls a day- i answer and it hangs up. i ignore it-goes to voicemail-it hangs up. i call it back and it is listed as disconnected or no longer in service ????? how does this crap work ?- how do i stop this without having to spend money to add call blocking to my phone?

same number and same problem.  would love to know how to stop this call.