This has not happened to me, but apparently it is a fairly common iPhone problem. You make a call on your iPhone and get an error message stating the “call failed”. I can see where that might be a bit annoying to the caller. If this has happened to you or is happening on a fairly regular basis, then read this article which lists ten different things you can try to fix this issue. The article is fairly complex, but worth your time if you are experiencing this problem.