Collection "Agencies" (CA) (not an actual gov't agency, but actually just a private firm) typically get paid a portion of any fees they collect. You don't pay thru them?... they don't get paid. In some cases, an original "creditor" will sell troublesome accounts for pennies on the dollar to CA's. CA's can care less whether or not you actually owe the money. They get enough people who pay something that they make money on their 37-cent stamp. If the CA can't collect anything...they'll discount the account and sell it off to still another CA. And the beat goes on.....
It's important to know how a "credit report" works, in such cases.
Your credit report is a record of
payments. It's not a record of non-payments. If a creditor says you owe them money, but have not paid, ...and IF they report it to the
credit-reporting agency (usually EquiFax, TRW, etc.... not the same as the CA), then you have to dispute the debt with the
reporting agency to relieve your report of negative value. The "creditor" then has to prove to the reporting agency that the debt is valid. (In this case, your letter to the reporting agency detailing the INvalidity of the debt should remove the item from your report.) But actually telling the CA that you don't owe the money will not relieve you of future threats. They'll simply let the file die...or not...and sometimes will actually "sell" your account to yet another CA...who'll in turn hope to harass you into sending some sort of payment. The beat goes on....) Unfortunately, in this case, Premium may have quickly sold off the acct to the CA who now considers themselves the "creditor". It may get convoluted quickly. (It'd be a lot more convenient to get it stopped now, if possible.)
Sometimes a CA will offer to "settle" for a drastically reduced amount, especially on an old, disputed debt. If the unsuspecting actually makes such a payment (presumeably to get rid of the calls/letters) then the credit report will actually be damaged...not helped, by the payment. This is because the report will then indicate that the debt was "valid" (or you wouldn't have paid anything) but that you took forever to pay and you didn't pay the full amount owed!

To top off the insult, some CA's will then send you a tax form at the end of the year which indicates your "debt" was forgiven, and now you owe the IRS taxes on the "forgiven" amount of the debt! (They in the meantime, took a tax deduction on money you never owed.)
I discovered some of this thru a divorce years ago, when my ex, with a pocket full of my credit cards, applied for new cards based upon her "credit" history of payments using my cards while we were married. Since we were not yet divorced, her name was still attached to my credit file. When the divorce was final several months later, she abandoned the accounts, and here came credit card invoices for charges on cards I knew nothing about, for unknown purchases made after the separation. I refused to pay them, and told creditors to collect their money from the person whose signature was on the charges. (Since she had no job living with her boyfriend, they wanted to go after whoever they thought had ability to pay. They didn't care that they were charges I never authorized on accounts I never owned. The accts were opened while the marriage was still valid, therefore....)
Their response was that they'd ruin my credit if I didn't pay, and the accts were passed from CA to CA thru the years. My lawyer had me refer their calls to him, and he'd tell them to produce evidence of "original indebtedness"...which never implicated me at all, of course. After a few years they all finally gave up. My credit report never shows any of this, despite all their threats, because I never paid them anything. If I'd paid something, then I'd have been shafted. (My attorney told me of a client who made the mistake of paying off his ex wifes cards to stop harrassing calls which were upsetting his new wife. He not only had derogatory late-pay records placed in his credit report, he also got to pay IRS taxes on forgiven debts.)
I'm not a lawyer, so this isn't legal advice. (And the law has changed since last year when this congress changed bankruptcy law and made it harder for common folk to deal with such problems. Don't know how valid my past experience is in the present environment.)
I'd recommend you send the AG, the CA, and Premium certified letters (return reciept) detailing the problem and demanding Premium respond directly to you. The credit reporting agencies (3 of them) must give you a free report and must include a copy of your letter in your report if you submit it to them. (I suspect Premium is claiming the debt as valid since you paid a "portion" of their invoice, and they are claiming the rest as "outstanding". Even if they've only made an error because they have lousy service and lousy bookkeeping,... it causes YOU the trouble of correcting their errors!)
Let this be a warning to anyone to stay away from them. Let 'em go out of business.