Two choices for cheaper international calls with an Italian SIM - also get either a calling card or callback service, or both. Some calling card access numbers are recognised and blocked; I'd be interested to know about the Bizon ones which would charge 2c on top of the SIM call charge (Wind ?0.10)
Another choice for reaching UM cheaply, only just launched, is the Finarea VoIP company
www.internetcalls.com - call charge 10.44c ?, about 12.5c US - the very interesting part is call forwarding and an incoming number (not US yet though), which means it's usable without an internet connection. But note - it would only be worthwhile if you expect to use it quite a bit for chargeable calls, as the initial credit will expire in 120 days