Visa should be fine. The exchange rate is handled automatically.

You don't HAVE TO use computer to make a call, but the fees would be a little higher, which means you might not be able to make 120 calls per year.

One of the way is to go to their web site, put in the number you want to call, and the phone you want to use to make the call. You would hear the phone rings, and when you pick it up, they will dial the destination number. The fees is about 8 Euro Cent.