According to my lovely wife, 12593 connects you to some sort of IP2Phone interface and offers lower rates for long distance than the standard bargain-basement phone plan. She doesn't use it since she has one of the annoyingly expensive phone plans that includes a pile of free long distance minutes.
For calling back to the US, the only way to be sure is to call the number directly, then to try it via 12593. Check your balance, then call a voicemail number somewhere back in the US dialing directly. Hang up after it answers, recheck your balance and repeat with 12593. Then you'll have an idea of what the rates are like.