Number called my cell. Didn't answer. Let it go to voicemail,  and no message left.

Gotta laugh...caller id says Diversified CON...yes, con. I am sure that is indeed what they do.
The con for today is they represent the Internal Revenue Service and are suing me for back taxes. Offered to "make a deal" if I get a green dot card??

I have received two calls today alone from this number. No voicemail left so I don't answer. I don't answer the phone if I don't recognize the number.

They have called 3 times so far since yesterday. I don't answer unknown callers and let it go to voicemail. They hasn't left any messages and I'm getting annoyed

They're not likely to leave you anything, there's little to gain by it unless they're bogus IRS agents or financial scammers. It's a live voice they're after most and the calls are liable to continue if they're getting something. Get on their nerves by giving no kind of answer to eventually discourage them from calling you.