Any call to a telephone number with the 800 prefix in Spain is free since the cost of the call is shouldered by the entity that receives it, according to the Organisation of Consumers and Users (OCU). But it is a very different matter with numbers that start with 803, 806 and 807, the higher the next digit, the more the call will cost.
Prefixes 803, 806 and 807 are reserved, like 905, for phone lines where a "service is provided" during the call. For example, 803 is used for adult services; 806 is for entertainment services such as tarot or games; and 807 for professional services.
But what can really raise a phone bill significantly is the fourth digit of these numbers. If the fourth figure is zero or one, calls from a mobile cannot cost more than 65 cents per minute.
If the fourth digit is higher than 6, the customer must have expressly authorised their telephone provider permission to access this type of call.
These calls are "ultra-expensive" with a minimum outlay of 1.30 euros and costing 1.30 euros for every additional minute, and some companies may charge even more than that, the OCU said.
Another prefix to be especially careful with is 118 as the costs associated with these calls can snowball, according to the Ministry of Energy, Tourism and Digital Agenda. Under new regulations, providers are required to warn customers if the cost of a call is going to exceed 2.50 euros per minute.