iPhone international sms send issue

In the last days I had an issues with my iPhone 5 and cross country sms. I was unable to send sms to a an international number with prefix. The phone always reported an error. After a lots of attempts I discover that the problem was the Dial assist, you can disable it under “Settings -> Phone -> Dial assist“. I’m in europe and the iOS manual report that this function is for US.

Reference:

http://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf