Scumbag collection agency that buys old debt that cannot be legally collected then continually harass you. Bottom feeders.
To get rid of them for good, ask for their mailing address. By law they must provide this. Next send them a letter by registered mail (type they have to sign for). In the letter state " You dispute their claim and if they wish to continue they can take the matter to court. All calls to me are to cease". After receiving the letter by law they must stop calling you, and they will. Don't worry, they will not take you to court.... On the small chance they do not respect the letter, report them to The Ministry of Consumer Services. Or heck, just report them to the Ministry for the heck of it, to make their life difficult.
DO NOT FEED THESE CREATURES!!!! They have no grounds to collect from you, you DO NOT have to pay them. The only reason they survive is because they trick people. Don't pay them, they will go broke and have to find a self respecting real job.