Although i use Dialwise on 08442001010 for calling spanish land line from uk 1/2 pence per min
I have found planet very good value when calling a spanish mobile from uk land line 5 pence per min
http://www.planet-numbers.co.uk/phone_spain.jsp (http://www.planet-numbers.co.uk/phone_spain.jsp) ;D ;D
One thing with the sites using 0844 or 0870 numbers etc is if you do not use BT as your phone supplier then check the call charges for these numbers.
Virgin, for example are quite expensive for so called 'local rate' numbers.
I use Call to Call on 08449092012[/u] costing 6p per minute plus a 16p connection charge with talktalk. It could be cheaper with BT as it quotes 4p per minute.
I did use Dialwise but it often failed to connect but still charged 19p connection charge.