a returned equipment (modem) charge?

I returned a modem back in July 2017 in the box it came. One was taken by the service technician who came to our house. I had only had the modem I returned about 3 weeks if that. The previous one had issues and this one wasn't any better. So I cancelled and returned it. So after a while I checked the tracking #. It says delivered. Ok great! I put the receipt away and forget about it. Then I start getting calls from a strange number in November-December. It's a claims company! Imagine my shock and outrage. This is so sneaky and underhanded. Wait till your prey is no longer expecting you then attack?

And the same taunt, oh if you had your tracking #, you'd be ok is so aggravating. I've called numerous times and it's always the same song. Nothing we can do, if only you had your tracking #. I've asked them to search the warehouse twice but an employee told me on my third try that even when they put in the search requests sometimes they don't even look at them and the requests are returned almost immediately has "no equipment found" so asking for a search is useless.

It makes no sense to me, if I left because of slow, inconsistent service that AT&T's response would be to harass me months after the fact. Well AT&T has a denouncer for life. This whole experience has a left a bad taste in my mouth and I am really disappointed. 

One thing I know, I am not, NOT paying that $150. I refuse to allow any corporation to take advantage of me like this. I get free legal services and am taking AT&T to small claims court since that is all I can do. Because it might affect my credit now but not forever so I refuse to duped and frightened into paying this extortion fee for fear of it damaging my credit.

[Edited to comply with Guidelines]