I have this same error when trying to use forms in the U.S. I've tried various combinations of putting "+" and "1" in front of the server's phone number but it hasn't worked. Has anyone gotten it to work in the US? If so, please let me know what you put for the server's phone number.
Thanks for your help.
Just to add here too, best thing to do is first check that the device you are using is compatible with FrontlineForms, as shown on the list found here: http://www.frontlinesms.com/resources/frontlineforms/
If you have further questions once testing with a compatible device please do feel free to start a new thread on the forum.
Thanks, and good luck with the testing!