See this thread from last year on this.

I don't know Spanish landline tariffs, but I believe that for callthrough providers you have a choice between access numbers slightly above ordinary rates, a bit like 0845 here, which connect the call without an account, and geographic or freephone numbers which are used on account-based versions. There may also be carrier-select prefixes; I don't know.

I'd suggest you go on the home phones board of the Moneysavingexpert forum, and do a search for Spain, as I'm sure I've seen the subject come around from time to time and there are others besides that Direct Telecom

If you also make calls from a mobile, then Vodafone's 60 for 1 minutes option looks quite interesting to use with a local calling card or similar access. Or HappyMovil and Lebara have cheap direct-dialled int'l calls, though I notice Spanish mobiles like having a chunky connection fee