I work for a marketing company myself, not sure if the Economist is one of the magazines we call out for. But basically, how we operate is we have a database on our computers that is share with us from a magazine's publisher. It is then our job (we're literally the middle men here) to ring each and every number of the publisher's behalf gathering subscriptions.
Once we get to the end of the database, we go back to the start and work through again. We have to keep going until either our deadline hits, or we hit the ammount of registrations the publisher was looking for.
Basically, if you get these calls, tell the person on the other end and if they're doing their job properly, they're mark you down as so and you should stop being pestered. If someone puts you down for a callback, then they're job is on the line for falsifying records and the company could get into huge trouble with the audit commission.
Hope this helps people!