Do a police officer need to identify themselves if they are call for some chargers?

So if they called and said you will be charged with something that you did online do they have to Id themselves cause I got a call once and they left a message and then I got another call 5 mins saying did you get our message I replied I don't know who this is I don't pick up blocked numbers and they replied with oh that's how it is well we'll be going to a judge.