Once again, David and I are aligned almost like twins. I think Fring is cool, but I question its utility, especially in GPRS and even EDGE markets where the audio sounds so muddy that its not what you would want to use for a business call.
David also experienced the same SMS issue that I did on a USA SIM. I swapped to my French SIM while in San Diego, did the SMS thing and voila, I had Fring running so I contend it may have been a carrier blocking issue.
So give David's post a read as it is like most of what he rights, right on the money.