My first reaction was to say that's wrong. Maybe it's just new, but last time I checked, a couple of years ago, there's no way to get a decent land line under 40€. The trick was that the "cuota de línea" is hidden from the listed price. Most of the land lines are owned by Movistar that rents them to Vodafone thieves and other ISPs that consider it an independent expense, as if it's not just a regular cost. Another possibility are time-limited offers.

Now it's my landlord who pays so I don't care, but still would like to know where did you find that price.