I am a EUROCITY fan, also. 120 minutes for €6 to either the States or to W. Europe. Anytime and from any phone. You dial the 900 number, enter the secret code, dial the number and go. As Chica indicated, the only drawback is that you have to use the card within 30 days of first use. An excellent value. Not all kioskos carry it.

You can purchase a pre-payment card in any "estanco" as madridmanjim says. But those are Telefonica cards and they will only work at Telefonica payphones.

There is a place in C/ Montera, where you can get also pre-payment cards through another company. Those don´t come with a chip to be inserted in the payphone, but have a personal code to activate them and then you dial a loooooooooong number to access that company and call your family.

Those are really cheap but I remember using them once and not getting a really good connection.

I only mention EUROCITY because it is the cheapest card I have found. As I indicated in my previous note, it can be used on any phone and used anytime. Period. Dial the toll-free number and the "secret code" and then dial up the number. Simple. 120 minutes for €6. Only rarely have I ever had problems with line quality.