Complaint Review: Bureau of Collection Recovery LLC

So I moved in with my GF recently and as part of our moving in together, we got a family cellphone plan with Sprint. She had ATT, so we cancelled it, and paid the early cancellation fee because of it. Total was about $200. I remember the figure, because I threw a fit over how much the cancellation fee was. Her monthly payments were about $45. So, add, $130+$45=$200 ish.  She never got any other communiqu from ATT again, save for promotions via email. Not one statement, not one past due bill nothing. Sure, she had her billing as paperless, but still, when a company wants to get you, they get you. I work in a call center, I know for a fact, when I can't get someone on the phone, you send a letter ASAP.

Today I get a letter from Bureau of Collection Recovery LLC stating we owe $259, to ATT Mobility, and the collection fee of $46.74 brings it all to a whopping $306.42. That would mean she would have been able to keep using the phone...well, we cancelled it in August, so, until today!

Now, BCR is offering to knock back 20% of the amount due to settle.  And this is where I knew this was a scam.  If ATT wanted $300, they would charge it all. Not compromise.

I got this notice today and will follow up with ATT tomorrow, to inquire about amounts due, but I don't know what to do with the notice. This is a first notice, thankfully they haven't called.  What should or could I do to nip this in the bud?