(Note - comments are as to Virginia law - probably but not necessarily the same where you live)
If you owe money to A and get a bit behind, you may get a call or letter from B saying "you owe A this much, send us (B) a check at our (B) address".
Caution: if you pay B you don't necessarily get credit towards your debt to A. B may be entitled to legally keep your money without having to send any of it to the creditor to reduce your debt if you signed a contract that says you're willing to pay all costs of collection (a standard boilerplate provision in most consumer contracts). Secondly, it could be a scam - even if there is a relationship between A and B, B could still defraud you and A could go to court and get a judgment against you - your payment to B would not be a defense.
Always, Always, Always make your payments to that person (including corporate entities) to whom you actually owe the money unless and until you get something in writing from the original creditor indicating that they have transferred or assigned the debt to someone else and instructing you to make future payments to that someone else. Never ever send money directly to a collection agency without having first received such instructions from the creditor.
That means, do not disclose the details of any payment medium to anyone over the phone, especially if (1) it's not someone whom you personally know and trust and (2) you called them, they didn't call you. That includes checking account and credit card information, paypal authorization, etc.