Just noticed that upon doing a topup, Lycamobile are now offering free calls for one month to other Lycamobiles in any of their countries. I think this applies to any of their countries.
http://www.lycamobile.co.uk/draft1-en/Lyca-to-Lyca1
http://www.lycamobile.es/en/lyca-to-lyca
As Lycamobile SIMs are amongst the easiest to pickup on arrival in lots of European countries, this could actually be quite an interesting product for roaming. For other calls, their call charges are not great, but they usually have decent rates for calling local landlines so you can tie one together with a Betamax/Dellmont local access number for free/cheap international calls.
Apologies if this has already been posted (I looked... but couldn't find anything)