I don't know all the specific details of your case, so I can only speak in general terms.

Keystone tried this crap with me several years ago, claiming I owed a small amount of back taxes from several years' prior - just far enough back that finding the paperwork proving they're wrong would be a pain, but for just small enough of an amount that the avg. person would cough it up to be done with it. This is a money-making ploy. I'm not sure if that's what's going on in your case, but it wouldn't surprise me.

I attempted to call their office - all my calls went to voicemail. I attempted to find a physical office, but the only info I found (at that time) was a PO Box (you can now find a local tax office here). I did notice on the back of the bill they sent me was a blurb about my rights under Fair Debt Collections (or some such blurb - check if it says something along those lines on any correspondence they sent you, because it's legally required). So I sent them a certified letter basically quoting their own legaleze, requesting they prove I owed the money. This site has some good sample letters you might be able to use. Include a request not only to prove the correct amount you owe, but also request an itemized bill of all the penalties they claim you owe and request how those fees are calculated. Per the 2004 PA Act 192: "Costs related to the collection of unpaid per capita, occupation or occupational privilege taxes may only be assessed, levied and collected for five years from the last day of the calendar year in which the tax was due."

In my case, Keystone figured I wasn't worth the effort after receiving that letter. I did get some small satisfaction knowing they probably paid more money in postage & to have someone type up that bill than the measly amount they claimed I owed.

If you still receive threatening letters/bills from Keystone, then you may have to hire a CPA or someone to deal with it. But do not pay that bill until THEY prove you owe the money.