Yes, it's surprising how large amount of the SMSs get lost. At least in Finland this has been the case when using the most popular international SMS API providers. I previously thought also that eventually the messages will be received but have learned otherwise after trying several providers for two different apps. And I have not even found any good explanations for the SMSs getting lost. I think this is sad from the perspective of Progressive Web Applications because often SMSs could act as an easy replacement for push notifications (which are kind of unavailable/unreliable for PWAs).