I've been involved in trying to create services and authentication for two web applications with the help of SMS API services from several of the largest international providers of such services. It has been amazing to see how large portion of the authentications SMSs and other messages do not reach the receiver at all. Some numbers don't seem to be reachable at all and some numbers can be reached most of the time, even though the numbers are totally valid mobile phone numbers. At least in Finland this is the case when using the international SMS API service providers.