Keystone collections are a bunch of shitbags and scammers. I've gone to war with them multiple times in the past. They will literally send you tax bills and collections notices, and threaten wage garnishment without a shred of proof as to your residency, and will essentially say "prove to us that you didn't live there, or pay up". I received several notices to this effect for different locales over the years - places I either never lived, or hadn't lived for over a decade. I sent them proof of residency for that time - they said it wasn't enough. I sent them my lease agreement for that time that showed I lived elsewhere AND the address change with the dates - they said not good enough. I contacted a lawyer and paid him $100 to basically proof my written letter, correct accordingly, and send with his letterhead. Finally that was good enough.

Then they contacted me the next year claiming a whole other residency and starting the process all over. This time, the lawyer sent a letter for free threatening a lawsuit for harassment and a bunch of other consumer reporting/credit/collection activity irregularities in their verbiage. Never heard from them ever again until I actually did owe taxes like 8 years later after moving.

All that is to say, contact an attorney ASAP. Nobody likes KCG, and they legitimately would be at risk of being sued if they didn't represent who they represent.