(My answer to a recent question...)
Obviously, if you currently own an unblocked mobile, preferably a dual-sim smartphone, you can just buy a sim card in each country and fit it to your own phone.
As regards to cheaper calls to the US, it is very difficult to assess what the costs of calls would be. As I said above, if you used a Portuguese mobile in Spain to call the US, you would have to pay roaming charges as well as the call cost.
Have a look at these guys who offer low-cost services in each of the countries you want are asking about:
In my experience, the best way to reduce phone costs in Europe, is to have a powerful wifi equipped smartphone with Skype or similar installed. Then, whenever one is near a free wifi access point, one has very cost-effective calling internationally.
I was recently informed about this company: http://www.poivy.com/
You can download an app to your smartphone and, like skype.com, use them to call world-wide at reasonable rates.