Nothing. A call to an 0800 number, or to another toll-free number such as an 800, 900 or 1800 number, is free for the caller in the country the number belongs to.
The business that owns the number pays for the incoming calls. With CallFactory, a toll-free number costs €49 a month (€490 a year) with every Customer Service feature, and incoming calls have their own per-minute rate, which we quote on request.
Why businesses use 0800 numbers
Because the call is free, customers can reach you without thinking about the cost. That is why businesses put toll-free numbers in advertising, on their website and on packaging, for sales lines, customer service and campaigns.
Toll-free numbers can usually only be called from within their own country. If your customers are in several countries, you can get a toll-free number in each of them and manage them all in the same dashboard.