How possible is it to get my phone unlocked?

We switched to Spectrum internet at my dad’s office and it includes a free cell line. I mentioned it to a friend and he said he had several iPhones in a drawer where he put them over the years after upgrades. He gave me an iPhone 11 last weekend and our Spectrum rep helped me get it set up late today. In the process, we discovered it was compatible but still shows as locked to AT&T. I messaged my friend and he said “That’s what I was afraid of. They probably won’t respond to me. It was a corporate device.”

Is this a paperweight? Or is there a chance I can get it unlocked and use it? If so, what steps do I need to take?

Thanks in advance for any help.