I haven't filed in years

It sounds like you have two separate issues. The IRS does not call people out of the blue. Therefore, if you are receiving threatening phone calls accusing you of tax fraud and saying that immediate legal action will be taken if you don't pay up, you are likely yourself the victim of attempted fraud: most, if not all, of those calls are scams. Report the call to the police and do NOT respond to them. As for your failure to file past returns: you ideally need to correct that situation sooner rather than later. Find a good CPA that has experience with tax controversy matters and have the CPA help you put together and file the returns you should have filed and pay as much of the resulting tax liability as you can as quickly as possible. Then, don't fail to file returns in the future. Best wishes to you.