In most states, this alleged debt would be outside the statute of limitations for contract originated debt (usually 6 years from day of default). Meaning that, unless you pay any amount to them, it is no longer a debt that can be pursued through legal channels.

With any debt collector or buyer, per FDCPA, they must stop calling if you notify them. Best way to do this is by a certified letter stating your name, your phone numbers, and that it is not permitted to call you regarding this alleged debt. Offenses here can net you a bounty in court, but worth noting most folks with valid debts, can't afford to fight the onslaught of robocalls from spoofed numbers, as is all too common in the trade

Requesting validity of a debt is also covered under FDCPA, but with some general stipulations. Legally, a collector or debt buyer is required to provide validation in the 30 days after the initial notification to you if requested in writing. Most will still send documentation out of this window if they have it, but it becomes less transparent under your civil protections for them to be mandated to do so.

Obligatory IANAL, but if you find yourself in a similar position here, doing a few minutes of research on your protections and common "gotchas" of dealing with these outfits is incredibly valuable.