Martyn is correct.
Could be a number of things.
There are companies who buy statute barred debts, which are debts which aren’t legally enforceable (due to age) unless you admit to their existence. In which case, the call is to scare you into admitting the debt so they can then legally pursue it.
Could be that they’re scammers, preying on the chance that some people will pay a small settlement ‘for an easy life’ to stop them sending the boys round.
Could be a crank, or a ‘friend’ who thinks he’s funny.
In all cases, the chances of someone turning up on your doorstep tomorrow are infinitesimally small. They’re trying to scare you into giving them money, it’s unlikely that they’re actually going to put any effort into it beyond front line phone calls and boilerplate snotty letters.
The line you need to take is to refuse to talk to them and insist they put it in writing, and then if they do write you need to reply denying the debt and demanding proof. If it gets to this stage I can help further.