Hey there! So first off, I’d just type “stop” to opt out since they gave you that option. Next, maybe check your credit report to see if it’s listed there. If it is, you could send a certified lletter to the debt collector and ask for proof you know, like a signed contract or agreement, statements, that sort of thing.
What happens next really depends on what they send back, if anything. Oh, and you didn’t mention which state you’re in, so it might be worth checking out your state laws some of them actually have better protections than the FCRA and FDCPA. Totally use those if you can!