If you are concerned by the legitimacy of a company, then you should contact your State Attorney General office and inquire if they are registered to do business in the state and licensed to collect debts where you live.
I can't see where a debt management plan is going to be helpful for you. The issue seems to be, for whatever reason, you've found yourself again in a situation where you living expenses exceed your income.
Now that you have paid, you are on a list of people willing to fall for the scam, the threats and demands should begin to accelerate. The scammers are probably selling your name around as a good target.
Now don't get me wrong, there are many good, fair, reasonable, and law abiding debt collectors out there who would never think of engaging in such alleged behavior. And while some may feel "all is fair" in order to collect a debt, there are limits.