But don't do this from a US-based mobile phone as there will still be roaming charges, even for a local toll-free number.
Speak for yourself. I have a T-Mobile phone with UMA. Any time I'm using Wi-Fi, the phone is "in" the US, at least from a billing perspective. I find UMA handier than Skype because you can also receive calls back much easier than with Skype.
Generally speaking, though, brp's advice is right. Don't use your phone overseas or you'll be soaked anywhere from $1-$5/minute. (Or get a T-Mobile phone with Wi-Fi calling.)